Of course, you know you won't get an unbiased opinion here because we'll just start fighting each other. As someone who disagrees with 98% of Trump's beliefs (and IS NOT an attorney), I don't think you can sue. What's the difference between any other fight between students? Of course, there are situations where the school failed to provide "reasonable amount of protection", i.e. A faculty member hear a kid planning to fight your son and purposely did nothing about it because of the hat. Now, in another twist, have your son worn that hat to school before? I'd expect doing so to cause a commotion amongst kids. Right or wrong, I'd expect the school to not allow him to wear it just because of this.