If this girl posted from off campus, on a public blog/website, then this ruling is way out of bounds.
I’ve seen other attempts by schools to regulate childrens afterschool/off campus behavior and that should be the parents and law enforcements responsibility. This ruling sounds like another case of slippery slope encroachment.
Go on a public website and call your boss a douche bag by name. Does he have a right to fire you? I’d say yes, so this is probably the right decision.