My best thoughts would be to alarm every exit except for the main door, like found on many restaurants emergency exits. Have auto locks on each and camera feeds on each. Inform student body that anyone opening the doors from inside would be expelled for a week. Main entrance could have a metal detector and armed guard. This way you control egress to the school and get a immediate alarm if someone trying to avoid the guard. Also with doors locked only for entrance, emergency exits are available, and video evidence prevents students from pulling pranks. Initial cost for hardware is there, but personnel costs are only one person.
My best thoughts would be to alarm every exit except for the main door, like found on many restaurants emergency exits. Have auto locks on each and camera feeds on each. Inform student body that anyone opening the doors from inside would be expelled for a week. Main entrance could have a metal detector and armed guard. This way you control egress to the school and get a immediate alarm if someone trying to avoid the guard. Also with doors locked only for entrance, emergency exits are available, and video evidence prevents students from pulling pranks. Initial cost for hardware is there, but personnel costs are only one person.