Of course its disruptive. That was the intention. No one stopped him from voicing his opinion. They just didnt want to hear it. I'm pretty sure if you showed up at a BLM event doing that it wouldnt be for long The same thing applies to those that did want to hear him. They should have found a way to make the disrupters stop.
I missed the part where the students were forced to attend. Did they attend willingly? Either way there are acceptable forms to protest - heckling a speaker is not one of them.
However, if a heckler did attend a BLM rally with the intent of being vocally disruptive - you don't have to listen to him, right?
You can appeal to the police to remove him for causing a disturbance but if you lay hands on him you might find yourself on the wrong side of the bars, as you should.