It's probable that a fair amount of this has been happening online, so there are records of it. Even if most of the kids are 6 feet apart in the same room (except the ones staying home for a few weeks because of a suspicious cough or a sick parent) they're going to spend a lot of time looking at the screen.

The first offense (rather, the first time the school noticed something) was very likely something the teacher only noticed when it spilled offline into tears and "they're picking on me!" If the victim (who is NINE. And outnumbered. Poor kid.) isn't fairly specific about what's going on, the teacher can miss that it's racial, and miss that there is a ringleader driving the bullying someplace exceptionally nasty. So half a dozen kids maybe missed recess and wrote apologies or whatever they do these days, and then went quiet and gradually re-escalated.

Second offense is now. I have a strong suspicion the victim went to their parents, who could call the school and say "This is racist. How long has this been going on?" Then the school looks into it and sees it's been happening for months, and wants to convince the victim's family that they take it seriously.