Continuity mistake: When Danny gets shot at the end of the film, his paper flies out of his hand. But the very next shot of him shows that the paper is still in his hand being held over his head while he falls backwards. In one of the following shots the paper is back on the floor, but it's on the opposite side it should have been when it flew out of his hand. It's all shown in slow motion and it's very obvious.
Continuity mistake: Toward the beginning, when the principal calls Danny into his office to discuss his Mein Kampf paper, he first glares at the Jewish teacher who exits the principal's office. As the teacher is walking past, the clock on the wall reads 8:10. But when the camera angle changes to show him walk around a corner, the same clock reads 8:20. Apparently it took him 10 minutes to turn the corner.
Continuity mistake: When the three African-American guys throw the white guy into the bathroom to beat him up, watch the guy who locks the bathroom door. In one shot he is several feet behind his two friends, with his back to the camera locking the door and in the next shot he is standing right there with them.
Continuity mistake: Watch Danny's right arm when he is leaning on the fence and watching the basketball game after getting out of school. In at least one close up shot of him, there is a leather strap or brace on his arm and in other shots there is nothing on his arm.
Visible crew/equipment: When Derek is entering the party with Seth you can actually see the shadow of the camera and of some crew members on Derek's and Seth's bodies. It just takes a sec, but I think it's clear.
Answer: It's quite simple: He was one of them, a fellow skinhead. In prison different groups stick together for mutual protection and comradeship, and as neo-nazis/skinheads would be fairly unpopular with the colored inmates, all the more reason for them to protect each other.
Twotall