What is a 21 year old doing in high school?
It says in the article that the thugs didn’t go to the school. This is a common problem with high school dances in urban areas, especially in gang areas. I have a family member who is a police officer (not in CA) who is often hired to work off-duty at dances just to keep these people away.
It’s odd that this school appears to have had so little security, given the nature of Richmond.