Just seen this - it was fantastic. Very, very funny, moving, thought-provoking - and beautifully cast. Frances de la Tour particularly stood out IMO, and Mr Irwin and the boy who played Posner (don't know their names).
Only two mild down points for me: it was rather a creaky transition from stage to screen, and the headmaster was a bit OTT, but maybe that's because the other performances were so brilliant.
Heartily recommended, Alan Bennett at his best.