After the last bell
The School is a different building once the pupils have gone home. The corridor echoes, the classrooms smell of chalk and floor polish, and the staff room is the one place with a working kettle. The head's office looks out over the playground, the science lab keeps its cupboards locked, the gym is dark and the canteen has been wiped down for the morning.
Teachers, governors and the caretaker all hold a key to somewhere. Every one of them had a reason to stay late, and the clues show where each of them really was.
How it plays
School boards are full of furniture. Desks, bookshelves, a piano and the odd globe block their squares, while chairs, benches, stools and gym mats can be sat on, so a classroom can hold far fewer free cells than its size suggests. The corridor is usually long and thin and touches many rooms, which makes a clue that keeps someone out of it worth a lot.
Start with the head's office or the staff room, the small rooms, and use "beside" clues around the desks: only a handful of squares sit next to each one.
