The root of the problem is not #BlackLivesMatter in this case. Most likely, white or non-black students would be causing the disruption, not the wearer of the shirt. If schools gave their students proper education about racism and harassment, classroom disruption could be avoided.
In *Tinker v. Des Moines*, the Supreme Court held that in order to prohibit speech, the school "must be able to show that its action was caused by something more than a mere desire to avoid the discomfort and unpleasantness that always accompany an unpopular viewpoint." **Q.E.D.**
the shirt is not in itself offensive. It does not say "ONLY black lives matter." It does not say anything negative at all. I don't see why other students would have a problem with the shirt, and unless all clothing with "political" statements is banned under the dress code, there's no reason to ban.