I'm 18, so I've got just as many rights as the next person. As were most of these kids, they were all seniors. In my school at least, most teachers won't stop a senior from anything, we can just leave during school, we will get a write up for it, but they can't stop you if you're not a minor.
Its not the fact that it was a black history assembly, it was the fact that he was telling kids that Obama is the next great thing. I think the kids who left set a great example that no one should be molded by being forced to sit through something they did not want to.
As for the military, those men who are my superior have earned my respect, I don't have a problem following orders given to me, but they have limits as well, its not just a do whatever I tell you thing, there are a lot of rules.