For those of you that say what the shirt had on it was unimportant, imagine this - a black student wearing a Malcolm X t-shirt and told to reverse it. Malcolm X was a convicted felon and black separatist; would his picture not be "disruptive"? Perhaps a Hispanic youth and a Che Guevara t-shirt? Communist murderer, very disruptive.
It's because of the targeted youth and targeted group that it becomes politically acceptable to attack, as there are two groups of people who are not only not protected from discrimination, but actively targeted by every level - smokers and gun owners. This was pure politics, and as pointed out, the student and hundreds of his buddies showed up in NRA t-shirts without incident, because the school knew it had screwed up. Yes, I believe the student's family could certainly file a lawsuit, and should.