It's not even that. It's a nervous
grimace, it's what teens will do when confronted with something or someone dramatic that they have no experience in. It's the default look they will get when they don't know what to do, what to say, how to react - boys and girls both will do it. Anyone who's had to work around a lot of youngsters will recognize what that is and why it happens. That's what happened when creepy old drummer dude got in his face - Sandmann's brain locked on how to handle it, so out came the grimace. I knew it the second I saw that. He's not some devious KKK guy, he was just being a kid and they're picking on him for it.