I agree that in public schools the threshold is much higher but there are also exceptions carved out of the “substantial disruption” standard. In Fraser the Court held schools may regulate “patently offensive” speech that occurs on campus to teach students the value of civil habits and “the boundaries of socially appropriate behavior.” Also I am pretty sure the J.C. v Beverly Hills case dealt with speech that occurred off campus.
Whether the speech that is the subject of this thread rises to the standards articulated in any of the First Amendment cases is another question. Most likely not. But I don’t think the OP is looking to litigate this matter in any event. I think she is focused on another avenue.
How about the teacher that was fired last year from a Texas elementary school for making what some perceived as a racist comment on her own FB page? Off school property and on her own social media account. So setting aside the legalities there are also social or professional ramifications for these sort of things which perhaps the OP is more focused on.