Many black people don't tell on their own people (that's a fact) so that's how they knew. For example, they were selling to three black boys and one white boy that day, they automatically knew who told on them.
As for the school bus. obviously that was their plan. In the video I noticed that those boys watched him like a hawk and at the bus stop, he was going to get off but they got him beaten up. When he saw those thugs in that bus, he'd be thinking "Oh shit, what are they doing here? Why didn't the school call police on them? Damn, they are here to get me. I shouldn't have told on them".
What I don't understand is that the driver let the thugs in when if he never saw them in his bus before. Maybe he thought they were new students. I guess someone will ask him that question," You never saw those boys before in your bus so why did you let them in?" Maybe he will answer, "I didn't pay attention to students getting in".