Laws about social media?

I go to a private school in Wisconsin. This school tracks social media accounts even when they are private and anything posted that is offensive towards staff (or obviously illegal) is punished harshly. I was wondering how they can see private accounts, if searching private accounts is legal, and if someone can be legally punished for saying something like "(staff member) is a d*ck". It just does not seem right to me that the school would exercise so much control over what non-illegal things are said. Thank you for your help