I would not call that a good explanation; I’d call it a bigoted explanation not based in reality, since they said that Mexican students are prone to violence over free exercise of rights in the USA. The First Amendment plainly states that the kids with US flags on have a right to do so.
In the late 60’s they made the precise opposite ruling in a case that involved high-school students who protested Vietnam by wearing black armbands at school.
So....does the speech have to be anti-American to be permissible? Or do threats of violence trump the First Amendment?