Sounds like two Catholic schools so this is a little surprising.
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Not really. 20 or 30 years ago most Catholic high schools in California were predominantly white, but not so any longer. I’m not sure how or why it happened, but most football and basketball programs at Catholic high schools (at least in So. Cal.) are now predominantly black. These kids come from the ghettos and ‘hoods surrounding Los Angeles. They appear to be pulling the best athletes from black neighborhoods (Compton, Long Beach, etc.), and it’s to be expected that the fans are also from the ghetto/’hood, including gangbangers. And in most instances, you have not just rival gangbangers supporting opposing teams but also supporting the same team since private schools pull kids from different neighborhoods. They might be on the same team, but they are still enemies.
I recall in the 80s, 90s, and early 2000, private Catholic schools like Mater Dei, Service, and St. John Bosco were still a bastion for wealthy white kids, but that has changed over the past 2 decades. So much so, that now they compete in their own conference comprised of private parochial schools only.
Take a look at the rosters for Servite, Mater Dei and St. John Bosco and you will see what I am talking about.
Serra was Tom Brady and Barry Bonds high school