The BAOSN video had the gesture in question edited out, probably because of the network's partnership with Serra. I also heard those rumors about what happened to the visiting locker room at SI. I do know that both sides will accuse the others of racist taunts, which is something that can't really be confirmed or denied.
The "SI rejects" chant was in response to a "Serra rejects" chant. When all the stuff after the game took place, most of the Serra players celebrated with their team and fans, as one should after a 20-point comeback in a huge rivalry game. Storming the court and going nuts is completely warranted. Most of the Serra players celebrated appropriately, and the SI players did a very good job of acting professionally, waiting for the celebration to subside and shaking hands before heading to the locker room. A few people acted out.
When McDonald hit the buzzer beater in 2017, the fans appropriately rushed the court, and they did so without incident. A couple of players got knocked over because there were just so many people running, and one of the refs got inadvertently shoved, but it was nothing malicious. What a few of the kids did here was deliberate, and I'm still curious as to which of the rumors were true regarding what happened at the Beach Game to spark things.
I also know that the student sections from both teams were sent through different doors afterwards, but they reconvened in the hallway and outside the school as well. I was told there were a few punches involved and police had to be called to break things up. It wouldn't be the first time this happened. The link I was searching for seems to have expired, but in 2012, when SI pulled off a stunning 57-54 upset at Serra, the student section yell leaders played a game of knockout at halftime that turned into a physical altercation.
