Get the pro in, our air con failed last summer and I had to work in the 35 degree plus shuting everything down ops: while waiting for the same day service contractor to come it. Found out some kid had got on the roof and turned the maintence switch off (now encased in locked box).
I hope that every member of staff has a key as I was told under H&S and Fire Regualations that you have to be able to get to those isolators in an emergency, and fumbling round for your key or the person with the key / code is not acceptable. We have had the same issue of kids turning isolators off, and we put a padlock through them so they couldn't be turned but when we had a H&S Inspection they were flagged up as an issue to look at but not dangerous, but the local fire officer demanded they be removed ASAP and put in his report that they were dangerous locked on like that as if a fire occured if they were still running in the room with fire they could help spread the fire to other parts of the building, roof spaces etc so we had to unlock them and we asked about cages but they said you have to be able to reach in and turn them off through the cage, but that defeats the point of the cages. Touch wood nothing like that has happened yet, but there is still a few weeks of term left....
depending on how likely you think the kids are to turn it off again, you can get an environmental monitoring box for ~¬£200 (temperature sensor + one other sensor) that will email or SMS you if the temperature goes outside a range you define.
Blackbox have them available and most air-con installers will know of a similar solution.
I am having a re-vamp / move and ive got an APC environmental Monitoring kit kicking around under my desk from a mate so thats going in as well as the isolators are moving from the roof to a wall which will need Ladders to reach, which means they are kid proof but still accessible if needed as the firemen will have ladders if they need access.