I also agree you should get a cage with locks. AP and Boaphile both make nice PVC enclosures with locks. You might think it is overkill, but you can't trust everyone unfortunately. It isn't about the safety of others as much as it is the safety of the snake. Having a snake on the loose is a very frightening experience for a snake owner. I only had one escape and it was for less than one day. I wouldn't sleep until I found the snake. It was very stressful for me and not something I would want to go through again.

Put yourself in a position where this will never happen again and trust me you will be much happier and so will the snake! Get that keyed lock!

The snake will show up eventually. Are you allowed in the school at night to sit and listen for the snake moving? Its very possible that it has found a nice spot to curl up already and won't be moving anytime soon.