There's nothing subtle about Big School - for instance tonight's episode starts with a fart gag. But maybe that's why audiences enjoy David Walliams and the Dawson Brothers' cheerful, unsophisticated creation. Aged gags are telegraphed and thoroughly milked for every single comic possibility. Like the unfortunate pupil at Greybridge School, whose mother has run off with a Masai warrior during a family holiday. Cue sniggering staff making pointed and very lewd remarks (particularly grotesque headmistress Ms Baron) as thick Miss Postern ties herself in knots trying to be politically correct.

Elsewhere, Mrs Klebb presents the school play Juliet and Romeo, her "gender re-imagining of Shakespeare".

Eschewing the option of belting out Just Say No during a hastily scheduled assembly, Mr Church plumps for a more clandestine approach to tackling a nascent drug problem at Greybridge. Elsewhere, the relentlessly ditzy Miss Postern suggests the chess and draughts clubs be combined into a single, hybrid, board-based discipline. Three episodes in and still no heavier on laughs, with no let-up in the supergroup of supporting players being wasted on lazily conceived, one-note characters.

The chemistry experiments that open each episode of Big School show how combinations of some fairly innocuous looking elements can produce unexpectedly spectacular results.

And the unlikely love triangle that throws David Walliams, Catherine Tate and Philip Glenister into a test tube is still delivering plenty of bangs for your buck.

The culmination of this week's episode finds the desperately uncool Mr Church and the not-as-cool-as-she-pretends-she-is French teacher Miss Postern perched cosily together on her sofa on a Saturday night watching Strictly.

"Has the one on the end got a wife?" Mr Church inquires.

Also this week, drug dealing has become a problem at Greybridge School - a chance for all the teaching staff to unite and tackle the problem in their usual, ill-informed way.

And that means another duet.

Big School is unapologetically old school in its comic approach - it is currently BBC Comedy's holy grail to find a popular, mainstream and peak-time sitcom - but I was pleasantly surprised by how entertaining and funny it was.

Nobody is ever going to die of laughter while watching Big School, but creating characters an audience wants to spend half an hour with is the bedrock of all sitcoms, and these are well drawn, good fun and beautifully played by an illustrious cast that resolutely resists the temptation to do 'funny acting'.

David Walliams - who shares a writing credit with the self-styled Dawson Brothers, who were presumably leaders of an outlaw gang before turning to comedy - stars as chemistry teacher Mr Church, who harbours unrequited feelings for french teacher Miss Postern, played by Catherine Tate.

Walliams is sweet and funny, but pitches Church at the asexual end of camp, which effectively prevents any romantic chemistry developing between the couple and drives one of the show's major themes up a particularly blind alley.

It is also a shame that the kids at the school hardly get a look-in on the action - an oversight that effectively doomed school-set sitcom Chalk back in the 1990s - as it is common knowledge among teachers that the students are the funniest, silliest and most unpredictable part of any school.

The second episode of David Walliams' easy going comedy, bringing together an ensemble of recognisable faces, including Catherine Tate as the new, 'glamorous but knows it' French teacher. That man from Ashes to Ashes (Philip Gleinster), who plays the moronic 'geezer' of a P.E. teacher. That guy from the BT adverts and Black Mirror (Daniel Rigby), as the 'real music' fanatic, and Rising Damp star, Frances de la Tour, in fine form as the hilarious drunkard of a headmistress.

Each episode plays out like a short film, with ever predictable plot twists, low points and resolution. Textbook stuff. However this is done with an unexpected charm and warmth, simply due to the character stereotypes and all of their cringe-worthy glory. This combined with the staffroom perspective is a fairly original mix, matched only by Chris Lilley's Summer Heights High (which if you haven't watched I strongly recommend).

Whether this form will get repetitive is still up to question however, as there's only so many times you can watch a short There's Something About Mary.

So if you're gouging your own eyes at the thought of having to sit through another moody police drama (even writing this fills me with dread), and long for something as light and easy as angels delight while watching Sesame Street, then tune in to BBC One at 9.

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.

David Walliams's school-set sitcom limps on. Miss Postern is organising a talent show to raise money for Children In Need. So PE teacher Mr Gunn attempts his Keith Lemon impression, sad sack geography teacher Mr Barber sings a Welsh song, and Mr Church gets his oboe out. All the classroom tropes - the coarse PE guy, the hot new teacher, the boring chemistry teacher - are here, and not terribly funny. It makes you pine for Channel 4's far superior Teachers, from more than a decade ago.

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".