David Walliams' tedious documentary Snapshot in Time, in which he reunited old school chums, might have given cause for concern about his sitcom, entering its second episode tonight. But Big School is genuinely funny, the absolute highlight being Headmistress Baron, played by Frances de la Tour, who's brilliantly dry and has all the best lines. 'It had better not be a play about talking fannies,' she drawls from behind her huge desk before gulping from a glass of wine.

And Catherine Tate's not bad either as French teacher Miss Postern. Is it just us, or is she getting less outlandish (read: annoying) the more she's on telly? We approve. Miss Postern's unfounded haughtiness is snigger-worthy, especially when she's referring to mega pop star 'La Femme Gah Gah'. You see, this week the school is running a talent show and the teachers are taking part, giving Walliams's uptight character Mr Church a chance to get his oboe out for the ladies. Not laugh-a-minute stuff, but there are certainly enough funnies to amuse.

We've all been burned too many times to take these things on trust any more. Still, the cast list of Big School (BBC1) couldn't fail to draw the eye, stuffed like a prize summer pudding as it was with juicy comedy names. Catherine Tate. David Walliams (a co-writer of the series). Joanna Scanlan (Terri Coverley in The Thick of It). Frances de la Tour. Frances de la bloody Tour!

All that promise means there's further to fall, right? Yes, usually. But this time it was great. Not perfect. But great.

The setting and the setup - teachers in an urban comprehensive who are lazier and less mature than the kids - is far from original. But Walliams, playing the head of chemistry, Mr Church, who is on the point of resigning until he has a coup de foudre when the new French teacher, Miss Postern (Tate), walks in, has here tapped into his warmer, more vulnerable, altogether funnier side. It's the one that makes him a wonderful children's author, rather than his colder Little Britain side. This performance, and his writing, gives the show good jokes and heart - which unless you are sure you are the next Seinfeld, generally works out best for all concerned.

And Tate's a genius. Her opening scene with Frances de la Tour's headmistress will keep me happy all week. "I love everything French!" she exclaims. "Not just the language. The history, the romance, the bread." I can't reproduce it here with just the paltry resources of the printed word, but it's the way she breathes the greatest passion into "bread" that sets you honking. And that's before De la Tour fixes her with a stare that's seen a thousand teachers come and go, and replies without expression on her face but an infinity of loathing in her voice: "Dirty people. Dirty country."

Look, you had to be there. But make sure you are next week. The second episode was trailed at the end of this one and trust me, you're going to want to see Mr Church dancing out of the dinner hall while raging at Miss Postern: "French is the poor man's Spanish!" Gold star.

I severely wanted to like Big School, if only to celebrate on the BBC's behalf their having launched two successful comedies in a row, the other of course continuing to be the sublime Family Tree. If so, it would have represented an almost unique triumph (yes, yes, but I've consulted my inner pedant and he reluctantly allows this) after a couple of years of embarrassing twock.

But I didn't. Despite a highly promising cast - Philip Glenister, showing he can "do" comedy; Daniel Rigby (the "annoying one" off the BT Broadband ads) showing he can actually "do" real acting, and rather good he is; the wonder that is Frances de la Tour, somehow growing increasingly sexy with age; Joanna Scanlan (the sublime Terri from The Thick of It) - it remained stubbornly written by and starring David Walliams, with all that entails.

Which is to say: too occasional mini-smiles leavening a fast succession of stereotypes, interrupted by a lazy cliche or three, shot through with embarrassing pieces of slapstick, most cringeworthy of which was the ancient teacher Mr Hubble going into an occupied classroom and opening his flies ("the loos used to be here..."). That was the savage low point; the highs were any scene involving De la Tour as the humourless alcoholic headmistress. This cast - and did I mention Catherine Tate? - surely deserves more subtle writing. But Walliams seemingly can't think but in stereotypes - I'm sure you remember even though I'm trying to forget the vile Little Britain, written in and somehow encapsulating the dark, dying days of New Labour.

Walliams has said it's "slightly subtler than Little Britain"; not the biggest of asks. So all the pupils - count them: all - are badly behaved, rude and street-smart. Mr Church, Walliams's character, drives an Austin Allegro, ho ho, and listens to Phil Collins, hoo ha. Alan Partridge it ain't.

I might watch another episode, if only because openers are notoriously ham-fisted, and there's a sparrow-flicker of interest over the Glenister/Tate/Walliams love triangle. But at this rate Walliams is in danger of being remembered only for the 167th fastest crossing of the Channel. Which would be no bad thing.

From Please Sir! to Jack Whitehall's Bad Education, schools are a magnet for the sitcom gang.

The latest to answer the ringing of the bell is David Walliams, taking half-term break from teasing Simon Cowell to play lovestruck chemistry teacher Keith Church.

The object of the bumbling Keith's affections is la belle Miss Postern (Catherine Tate), the flame-haired new French teacher who doesn't actually know much French.

With Philip Glenister as a randy PE teacher, Frances de la Tour as a mean headteacher, Joanna Scanlan as a lesbian drama teacher and a scandalously under-used Daniel Rigby, the cast is top-notch - even if the jokes are a little old school.

The public disembowelling of The Wright Way will have put a few comic heavyweights on alert about upcoming projects, but David Walliams doesn't have too much to fear where his new sitcom Big School is concerned. It may be a little light on jokes, but it's transparently good-natured, agreeably old-fashioned and with an adult cast so attention-grabbing that the pupils occasionally feel a little incidental to proceedings.

Walliams is Mr Church, the sad-sack chemistry teacher who withdraws his proposed resignation when highly desirable maverick French teacher Miss Postern (Catherine Tate) arrives to shake up Greybridge secondary school. Frances de la Tour's withering headmistress, Daniel Rigby's clueless music tutor and Philip Glenister's non-PC PE teacher all grapple over scenes to steal and prise some good laughs out of the sometimes slight material.

It's no Grade-A student, but Big School isn't expulsion fodder either - a decent achievement with so few new sitcoms worthy of a pass these days.

Vicious (ITV) exited as it entered, a high-camp frolic with Ian McKellen and Derek Jacobi hamming it up like there was no tomorrow. The unapologetic portrayal of old-school gayness got up noses in some quarters, cited as evidence that TV has still not embraced sexual equality. Queerly, to me, it felt like the exact opposite.

The existence of Vicious seemed to suggest that TV was comfortable enough in its own skin to not give two figs about only showing a politically correct approved version of gay lifestyles.

Freddie and Stuart were riotous throwbacks, bitchy old queens from another era, but in between the barbs and the one-liners, these were recognisable characters. To deny their existence would surely be the politically incorrect thing.

Taken on its own level as a tribute to the old tradition of West End farce, McKellen and Jacobi's Vicious was worth a cheap laugh, an added treat being the presence of Marcia Warren and Frances de la Tour, who really should have a show of their own. And how refreshing to have a man in his seventies tell his mother-in-law: 'You're old, you're eating buttons.' Now that's anti-ageism.

This is the strangest episode so far, as Stuart (Derek Jacobi) and Freddie (Ian McKellen) invite young Ash (Iwan Rheon) and his new girlfriend Chloe (Alexandra Roach) to dinner and then behave abominably towards her.

Trouble is, Chloe is lovey-dovey, airy-fairy, vegan and teetotal - in short, incredibly annoying - but before long the wicked pair have brought out her own vicious streak. I don't want to oversell the comedy, because a lot of it is lame, but the tone veers towards Joe Orton.

This is also the show to turn to if you've longed to see Frances de la Tour (Violet) handcuffed to a bed in Argentina dressed in PVC bondage gear. Any takers?

Stuart and Freddie are still sneering, naive neighbour Ash is again pouring his heart out - inexplicably - to the leering pensioners, and Frances de la Tour's Violet is deciding whether to fly to Hungary to perform "depraved sex acts" (something that, admittedly, breaks with this sitcom's painful repetitiveness). But then Freddie gives an ill-informed - and very funny - masterclass on microscopically small acting parts, apropos of his Downton audition: a glimpse of how this series might have turned out had the rest of the script been nearly as clever.

The histrionics are in full flow when Freddie goes for an audition on Downton Abbey, although his only line of dialogue involves potatoes. The notion of Ian McKellen being up for such a small role is in itself funny, and the scene where Freddie sits gauche young Ash (Iwan Rheon) down for his first acting lesson is priceless.

Freddie and Stuart live up to the promise of the title tonight. They're in waspish mood, not just to each other and Ash but also to Violet - Frances de la Tour getting the best lines. "A lot of acting is just good hair," she avers, while seeking solace for a failed affair with Christoph: "He'd fly me to Hungary once a month to do... unspeakable things."

It's amazing how quickly Freddie, Stuart and Violet, the tart trio at the sweetly sour heart of Vicious (ITV), have become as familiar as old friends.

Only two episodes in and it already feels like we've known them for years. Which of course is the case, because what this defiantly old-school sitcom amounts to is a masterclass in stereotypes.

That it scarcely matters is down to the impeccable comic timing of Ian McKellen, Derek Jacobi and Frances de la Tour, who make the absolute most of what, in lesser hands, could be thin pickings.

Last night offered mild farce in a department store involving a suspected affair and a running gag about a might-be dead dog. Every line was expertly squeezed for laughs.

The real joke, of course, is that Vicious isn't really Vicious at all - it's obvious that, not very deep down at all, they love each other to bits.