I just love the explanation to the young boys at the end, that they had made a choice to "join the mob" instead of going away. But remember at the beginning the cop was lecturing other kids that they shouldn't run when they see a cop. Only the black kids. White kids were standing all over the place and not a word was said to them about anything.
These clearly aren't gang members or street toughs. They're just normal young teenagers going to a public pool on a hot summer day.