It was more that they've shown many of the kids making racial comments to each other, and as youth tend to do they justify their own negative behavior with a he/she/they started it defense. In an earlier episode Mateo tells an African-American guy to stop talking to Evy and to "stick to his own kind". Perhaps in Mateo's mind he's fighting back, but I think the show is showing how people are projecting stereotypes, frustration, etc. onto each other. Plus, one can be defensive and racist at the same time. The point that the principal was making in telling Mateo "fine, since you want to do this your way..." is that even if he felt something needed to be done about racial bullying then there are better ways to respond than bullying back with greater force - otherwise you'll end up in a vicious cycle.
Like someone mentioned earlier, I think these B/C-stories at Marshall High are meant to be commentary on class, and to juxtapose the concerns and conflicts in an environment of limited resources and relative anxiety to one of more abundant resources and relative order or calm.