I was in a slightly similar situation the other day. I'm a male (white), walking to boxing class when about 0.75 blocks away, I see some kid getting attacked by... wait for it... 4 other kids. All women. All African-American.
There was a group of about 12-15 high school age girls and 10 of them were just watching 3 or 4 of them gang up on 1 of them.
WTF do I do here? I had the same thought process as many people in this thread. I could get involved, but realistically, there's absolutely no way I'm going to go try to break up a fight with that many people in it, let alone teenage girls.
I watched for about 5 seconds before I started to get my phone out to call the cops (which I also felt bad about b/c... wtf are they going to do... something needs to happen now).
Luckily for the kid, she managed to get away and they stopped pursuing her. But man, the feeling of helplessness sucked pretty hard.
Maybe I missed it... or maybe it doesn't exist.