1) Students in Detention (one reads Lord of the Flies).
2) Teacher Leaves (with mobile phones).
3) Students discover they have been locked in.
4) They escape from the room.
5) Dawning realisation that the whole school is empty and they are locked in.
6) The group breaks into 2 factions (good and bad).
7) One group goes in search of the mobiles.
8) The other group goes to the staffroom/canteen.
9) Introduce the sense there is a person observing/manipulating the situation.
a) There was a way out all the time
b) Only a short time has really passed and they encounter a cleaner
c) They develop “Stockholm Syndrome” and some of them want to stay trapped in the school
d) Rescued by outside forces