I find that a bit of a stretch. Maybe smiling is disruptive too. Or perhaps giving someone the finger is disruptive. Maybe holding a photograph of an aborted fetus is disruptive too. Just because you are offended does not mean the action, whatever it is, is not a protected activity.
Way I see it is, kids get into a squabble. Boys siegfried girl. Girl screams “STOP IT!” Send everyone to the Principle’s office and let G-d sort them out. Nazi salutes are not protected activities, any more than wearing KKK sheets, or giving people the finger, cursing, or chasing another kid around the hall with a booger on your finger, threatening to wipe it on him. The Constitution has very limited free-speech protection inside a schoolhouse, and keeping order amongst the little savages is a compelling enough state interest to override what little there is of free speech.