The children dose the staff-room kettle with alcohol and the staff all get drunk from their cups of tea. Heating the kettle would boil all the alcohol off leaving just hot water.
Although several Internet sources refer to Mr. Wakefield as the school's headmaster, he is specifically stated at the start of the film to be the DEPUTY headmaster - the equivalent of a vice principal here in Canada - who is running the school due to the actual headmaster's illness.
During Mr Milton's classroom scene one of the students can be briefly seen looking directly at the camera - this happens at around the 19 minute mark.
The only staff seen throughout the film are played by Hattoe Jacques, Joan Sims, Kenneth Williams, Kenneth Connor and Charles Hawtrey with Ted Ray as the head. but when Rosalind Knight, the school inspector enters the head's study for a staff meeting there's a stranger sat in the corner. He's not seen or heard of during the meeting or later.
At the end of the school play when things have gone drastically haywire, Kenneth Connor starts throwng switches on the control panel unit which visibly moves and does so again when Kenneth Williams trys to switch things off.