The kids were trespassing, simple as that, because they weren't supposed to be there. They did NOT commit vandalism, not by the legal definition. Suspension is probably excessive, but it depends on what is in the student handbook. If there were flexibility, then an in-school detention and possibly a Saturday clean-up of the campus would be plenty.
The custodian should probably be spoken to, but that should be the end of his part.
The school board member should be reprimanded. A group of students should NOT be allowed into the building to play a prank, harmless or not. This is a breach of the trust placed in the school board, and while it's not something to be fired over, it shouldn't just be laughed off.
This senior prank itself was certainly creative and pretty much harmless, other than it might qualify as littering. The problem is that too many senior pranks go wrong, and some go very wrong. Kids this age don't think things through very well -- I know, I was one once, and Tex Jr. was one very recently too -- and if there is an expectation that the seniors will play some kind of prank, then sooner or later there will be a disaster. It's inevitable, ant that's why I firmly believe that senior pranks, and other similar kinds of "traditional" pranks, have no place in school. Yeah, it spoils the fun, but then having speed limits on the freeway spoils MY fun, too.
And no, there were no senior pranks when I was in high school. Nobody wanted to be expelled in their senior year, and at the beginning of every school year the senior class was very pointedly told that any sort of class prank would result in expulsion of the prankers.
We still managed to have fun, too.
