There's nothing like a school trip to let the hair down and, as non-French speaking French teacher Miss Postern heads for France, with Mr Church chasing breathlessly behind - or chasing her behind breathlessly - this final romp with the staff and pupils of Greybridge School is a full-on ooh-la-la farce. David Walliams and Catherine Tate star in a comedy that makes Bad Education look the height of subtlety but a second term seems a certainty.

It's our last visit, for now, to hopeless Greybridge School as we eavesdrop on yet another of Mr Church's unimpressive chemistry experiments, conducted under the adoring gaze of moon-faced Pat (a silent Julie T Wallace).

But there's excitement on the horizon, at least for the staff, with a school trip to Dieppe, led by French teacher Miss Postern (Catherine Tate) who, oddly, has never been to France before. She doesn't seem able to speak much French, either, as her coachload of uproarious pupils and three male teachers arrive at their accommodation. What happens next is pure crude, rude and ribald French farce involving hotel corridors and gatecrashed bedrooms. And David Walliams has given himself a diarrhoea-related sub-plot that might make you feel queasy.

At last, the episode with David Walliams in his underpants - the comedy moment that's been trailed since this knockabout school comedy started its first term. There's a sporting theme as Miss Postern (Catherine Tate) sends rival admirers Mr Church and Mr Gunn into a testosterone-fuelled frenzy by donning a tracksuit for ironman triathlon training. They settle it the way men have done since the dawn of time - with a spot of wrestling.

It really ought to have been a total winner, containing as it does - via cast members Philip Glenister, Frances De La Tour and Catherine Tate - the distilled magical essence of Life On Mars, Rising Damp and 'am I bovvered?'. But it feels tired, flabby, a bit obvious and not that funny - certainly when compared with its younger, zappier, rap-soundtracked, groovily-edited rival Bad Education.

And therein lies its problem, I think. Viewed in isolation, Big School would make a perfectly agreeable half-hour's entertainment, with the running gags about Mr Church's frustrated chemistry experiments and the quite-amusing 'Don't mention the war' scenario, whereby the staff fail to deal sensitively with the boy whose mother is having it off with a Maasai tribesman. Watch it after Bad Education, though, and suddenly it's as lame and embarrassing as watching your Dad trying to DJ.

Possibly thanks to its big-name cast - including David Walliams, Catherine Tate, Philip Glenister - Big School continues to have the feeling of a show that is a long way from being as funny as it should be. In fact, there's a rather unattractively retro, 1970s feel to tonight's episode, in which staff are instructed to behave sensitively towards a pupil called Josh: after an African holiday, his mum has run off with a Masai tribesman. Frances de la Tour as the headmistress remains the only bright spark in a class of underachievers.

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.