In the last in the series of this old-fashioned school sitcom, Miss Postern is organising a school trip to France. Of course, they can't afford Paris, so they're taking a coach to Dieppe - cue a recurring joke of "will we see the Eiffel Tower, Miss?" - and the teachers scrabble for a place on the trip. Everything goes wrong in Dieppe: an inability to speak or understand French, underage (and legal but excessive) drinking, a dodgy tummy etc. This series has been a grower, but it's still not A* quality.

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.

Viewers have taken to Big School (co-written by David Walliams and the Dawson brothers) in droves and they are rewarded by plenty of these crowd-pleasing moments (come on, who doesn't want to watch Philip Glenister drag David Walliams from a set of wall-bars?). But the real scene-stealer is Steve Speirs (he was Ricky Gervais's unwanted "friend" in Extras) as sentimental, self-dramatising Welsh geography teacher Mr Barber. He's prepared his class for an important exam: "I'm looking for the geography teachers of tomorrow." Or so he thinks.

Meanwhile, Greybridge School is transfixed by rumours that the chemistry department's prissy deputy head Mr Church has slept with thick French teacher Miss Postern. The rumour was started by Cro-Magnon gym teacher Mr Gunn, Miss Postern's other suitor and Mr Church's rival. In a big slapstick set-piece Church and Gunn (Walliams and Glenister) wrestle - literally - for Miss Postern's affections.

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.

A teacher's view on Big School

The staffroom stereotypes may be spot on, but even David Walliams can't save a show that laughs at teachers more than with them, says Sarah Jones.

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.