A deputy head I knew used to keep two entirely separate faces. Off duty he was warm and relaxed, with a crinkly smile and dimples. Prone to hideously funny practical jokes, he was famous for leaving messages for new staff which involved them ringing a parent called Mr C Lyon. Naive victims found themselves having embarrassing dialogues with the long-suffering keepers at Dudley Zoo.

But as he exited the staffroom his chin would move forward, his shoulders would straighten and he’d become flinty-eyed and frosty. Woe betide the miscreant who thought it a larf to ring the fire bell or set the library computer’s screen saver to display a Sun topless model. In his teacher mode, no mercy was shown.

Psychologist Erving Goffman wrote in the 1950s about the need for people to have a frontstage and backstage to their daily lives. Frontstage is where we perform. It’s the show we put on for other people. And the more complicated the role people have to play, the more vital it is to have a backstage area to prepare for the performance and recover afterwards.

The staffroom is the teacher’s backstage area. The only people around are the other luvvies, and they know it’s all an act. So newly qualified teachers walk into their new staffroom and find the health education co-ordinator having a crafty fag in one corner while the head of English struggles with the Mirror crossword in another.

Most staffrooms are hopelessly untidy - which explains why very few schools allow pupils to ever set foot inside.

Newly qualified teachers often fall foul of this unwritten rule and, having recruited a helper from Year 7 to carry a couple of kilos of books, they will be half way through the staffroom door when a yell of “Not in here!” stops the young Samaritan in their tracks.

Not all the teachers will have succumbed to the kind of myopia which permits the sufferer to ignore the accumulated detritus. One school staff returned after half term to discover that an enthusiastic new appointee to the staffroom committee had taken the oft-stated view that “this place needs a clear-up” literally, and cleared every surface. For the next few days hysterical staff were rummaging in the skip behind the building for lesson notes, GCSE coursework, 3C’s maths project and the application forms for the geography post.

Staffroom coffee arrangements are often an indicator of the style and ethos of the school. The machine in the corner is the norm. This will dispense brown water, usually in paper containers which efficiently transfer the scalding heat of the contents. Real mugs, coffee jars and tea bags reveal an optimistic staff, who genuinely believe everyone will pay their way and take turns at washing up.

Some staffrooms have a cleaner, and coffee and tea are dispensed in real cups along with home-cooked goodies and a beaming smile. If you find yourself on interview in such a school, murder the other applicants.

Some schools - horror - have no staffroom. Teachers and pupils share a canteen and departmental areas are open to pupils to wander in. In the loo staff discussing the character defects of Smith in 2a are likely to realise he’s in trap one, listening.

With no backroom space staff are in role all day. These teachers have nowhere to hide and the effect on their home life must be deleterious. Perhaps partners keep a punchball or might there just be a higher crime rate near these schools as homeward-bound teachers push old ladies under buses and mug passing shoppers?

So, on the first day, don’t worry about whose chair you are in, don’t try to impress anyone and don’t worry about whose mug you have just borrowed. Sit down and chill out. It’s what staffrooms are for. But beware of phone calls from parents called “G Raff”.