Aren't David Walliams, Catherine Tate and Philip Glenister a touch too old for the daft teacher parts they play in this old-school comedy? Or is that the point?

Either way, this is an amiable, oddly dated chip off the old blackboard - tonight built around a talent show - but, what with Waterloo Road and Jack Whitehall's Bad Education (returning for a new term soon), the TV syllabus is teeming with shows full of rubbish teachers. Give 'em a break!

The comedy gets much broader and blunter in the second episode of David Walliams's school sitcom, as seedy and inappropriate gym teacher Mr Gunn (Philip Glenister) thrusts pointedly in front of comely new French mistress Miss Postern (Catherine Tate).

Gunn and Mr Church (Walliams), the buttoned-up science department deputy head, are locked in a battle for Miss Postern's attention while she plays one off against the other. But she's a bit thick (she's never been to France and you have to wonder, generally, at her level of French-speaking ability), falling apart in front of frosty, iron-clad head teacher Ms Baron (Frances de la Tour, whom we don't see enough of). Tonight the staff, in a bit of Walliam's Britain's Got Talent self-reference, organise a teachers' talent show.

It's encouraging to see that David Walliams isn't above taking a pop at Britain's Got Talent. This week French teacher Miss Postern (Catherine Tate) decides to hold a charity talent contest for the teachers - to prove how wacky and zany she imagines she is.

It's a heaven-sent opportunity for love-struck chemistry teacher Mr Church (Walliams) to get closer to her with a duet. But as Walliams gets his oboe out, they face stiff competition from Philip Glenister's seedy gym teacher Trevor Gunn.

Big School succeeds because the cast is more than capable of ­spinning comedy gold out of some very crude elements.

Frances de La Tour as headmistress Miss Baron is magnificent tonight on the subject of Pudsey Bear. And Daniel Rigby's po-faced music teacher Mr Martin finally gets a chance to shine.

Best line of tonight's episode? It has to be Miss Postern's description of John Lennon as "one of the main ones of The Beatles".

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.

School-set sitcom created by David Walliams, featuring a supergroup of supporting players. The action in this opener centres on timid chemistry teacher Mr Church (Walliams), hoping to engineer some chemistry between himself and new French tutor Miss Postern (Catherine Tate). Soon finding himself competing with prickly PE master Mr Gunn (Philip Glenister) for her affections, Church turns to his class for seduction tips. If this debut is anything to go by, this looks a spiritual sibling to long-forgotten 90s class-com Chalk. It's that poor.

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.

BBC Two's spoof crime drama, recently returned for a second series, includes a character who seems a close relation to Philip Glenister's Gene Hunt from Life on Mars and its sequel Ashes to Ashes.

But rather than spitting his sexist lines out with a Hunt-like swagger, Toby Stephens's DI Jack Armstrong just assumes that all lesbians are simply waiting for his obvious charms. Miranda Raison, as his wearied detective partner Georgina Dixon, was left to remind her boss that not all pretty girls, straight or gay, pine for a middle-aged copper.

Just as in Life on Mars, Vexed offered characters that were in their own ways compelling, plus an absorbing plot. Was the girl found dead in the library murdered by the supposedly lesbian lover of her male tutor? Was it really plausible that a university so riven with Sixties gender politics still existed in the 21st century? And is it a requirement that all professors of English Literature are polo-neck-wearing Lotharios?

There were times during the hour that this viewer wished for an actor of Glenister's vigour to speak Armstrong's lines. But by about 40 minutes in, the characters seemed rather more convincing than the comic clichés of Life on Mars. That doesn't make them half as much fun, but it certainly made it a lot harder to turn off without knowing whodunit in the library.