A university can try to punish you for things you say or do on Facebook. The question would be, are they within their rights to do so? They may or may not be. It depends on what's posted or said on Facebook. If it's sexual in nature, racial threatening, that's a certain consideration. If it's simply a matter of somebody disagreeing with somebody's opinion about, say, a political matter or social matter, the school is trying to discipline a student for that, the school may be outside its bounds, and very well would be in trying to do so.
The student does have a certain obligation to live up to the school's code of conduct and other policies that may be at play, but schools can definitely be overzealous in terms of trying to discipline students for speech and conduct that would otherwise be allowed. Having experienced attorney-adviser will help you understand and protect your rights and interest, and they can help you understand, navigate the process if you're facing a misconduct allegation of that nature.