Pranks like that can get you in serious trouble.
Pulling Fire alarm. -Illegal, unless there is a fire.
I doubt sprinkler systems in school are attached to some kind of network if they're it's local; not remote as in a specific computer only. (The school I went to anyway) simply way to trigger is to have a long lighter and find a spot for where you won't get caught.
but again, don't do it because depending where they're located it's going to cause more damage then anything including computers and that's when they start doing budget cuts.
and you can actually be tried as an adult and will be on your record.
Vaseline, someone might fall and hurt themselves, if someone breaks there neck and if they see you do; Consider it involuntary man slaughter and your life is over, because of a supposedly harmless prank.
Only one, I think that would be fun, can't get caught doing is simply maybe do something within the computer network; Lock it down and say a 'Message' or some sort, I'm not sure how the computer network setup as they're all different, but this is what I do.
Like for instance, I crashed the firewall at school, was down for awhile and all those 'Blocked Sites' became unblocked.