Reason to think that the wearing of the T-shirts would lead to disruption?
Check it out:

Generally speaking, it’s not terribly controversial to say that school authorities have something to say about what their students wear. When the authorities forbid wearing American flag apparel, controversy begins. And when the reason is because racist thugs are threatening to assault the people who wear American flag clothing, the situation becomes absolutely appalling.

I can’t think of any way to make the decision described by lawyer and blogger Eugene Volokh sound less awful:

Thursday’s Dariano v. Morgan Hill Unified School Dist. (9th Cir. Feb. 27, 2014) upholds a California high school’s decision to forbid students from wearing American flag T-shirts on Cinco de Mayo.