There are two points at issue - and I'm not talking about the specific case, because I don't know enough about it, and nobody else here does either.

On the one hand, is using negro as a Spanish word to be seen as offensive. And I doubt if anyone would say that it is.

On the other, is using it as an English word something that should be seen as offensive and grossly so. My view is that could only be so if there were other strong evidence that the teacher was behaving in a racist fashion, and the use of the word in itself should not be penalised.