In high school, I was an edgy little shit, and for some reason it irritated me that the last person to use my shared computer had changed the background to a Christian image (Jesus on the cross or something). So I went into MS Paint and made an inverted pentagram with "666" and "Hail Satan" on it, and made that the new background. It never occurred to me that there would be any consequences other than pissing off whoever else used that computer.

The next day, I was called into the principal's office along with the school therapist and the computer teacher and they told me they were suspending me for three days, out-of-school, for devil worship. I pointed out that devil worship wasn't against school policy or the law, and they conferred for a moment and changed the charge to vandalism. I told them it was ridiculous to call changing a computer background vandalism, and you could change it in five seconds, and they told me I was getting off easy and not to push it.

So that's the dumbest thing that happened in my school.