The case isn’t that unusual to me. The Supreme Court has consistently maintained the stance that K-12 students do not have “freedom of speech” while on campus, as outlined in some of the landmark cases here.
My school district in Southern California has a very strict dress code to curtail gang violence. My mom swears that pink tennis shoes were banned when I was in elementary school because the district said that they were being used by a nearby gang. If black and silver is permanently banned, pink shoes are temporarily banned, then it doesn’t surprise me at all that national flags would be banned.
As a side note though, Cinco de Mayo has always felt like a ridiculous holiday to me. I really didn’t have very many friends growing up who took it seriously. However, I attended predominantly Mexican-populated schools. I imagine the sense of national pride and protection of the holiday is different at a school with far more racial tension.
I do wish students had more rights when it came to free speech, but I can’t really fault the school for trying to handle this situation.