If what you say is true it is abhorrent.
For the school to brush it off saying “their kids should learn to expect it and be less “thin skinned” is a complete lack of sensitivity and very wrong.
One thing I will tell you though is to be pragmatic. No, do not tire of fighting to change the system so that such harassment not be tolerated. It absolutely must not be. But all the while keep in mind that kids will always be kids, and there will always be a kid who makes fun of the fat kid, or the short kid, the weak kid, or in your case, the black kid. It is a terrible thing, but it is a sad fact of life.