What about something that would put another student, or perhaps a teacher, at risk -- but the perpetrator doesn't realize exactly how much risk would be caused. For instance, putting hot peppers into a teacher's cup of coffee, thinking it would be gross--only to find out that the teacher is allergic to hot peppers.
Also, what kind of school is it, and where? I went to a very rigid private girl's school, and it was not all that hard to get expelled. We had a strict policy that allowed no "warnings" for stealing, and one girl got expelled for stealing candy off another girl's locker that had been decorated for her birthday. Another girl got expelled for calling the principal the F-word (in public). And a third got expelled -- second semester senior year! -- for taking a textbook from the lost-and-found, crossing out the name of the original owner, and writing her own name instead. So if it's a private school or something like that, there could be very specific rules that have expulsion as a punishment.