abide by the rules of conduct those students (who presumably can read) signed saying they understood and would abide by them. I'm not saying all of the rules are "just". Yes, some of them are silly and I didn't abide by them 100%, but I knew when I was not in compliance and shouldn't've or wouldn't've been shocked if I were to have at least been called into the office on them.

Don't conflate expulsion from school (only in extreme cases) with excommunication from church.