Case law/precedents are clear on this: Public Universities have a very high, almost impossibly high, bar to cross when it comes to expelling someone for speech.
“Hostile environment” may be an easy trigger to pull to get the punks off campus right away, but it’s going to be next to impossible to defend in court. Especially since the offending speech was made on a privately chartered bus going to a private function.
I disagree that the university will have a problem defending it in court. Violations of the student code or hostile environments can be defined broadly enough to protect them from suits.