"Achievement gap" contains no directly racially offensive language. It labels a scientifically observed and verified phenomenon that students of one race generally do better academically than another. That observation is racially defined but is not racist, simply because it is an independently verifiable fact.
Responses to the "achievement gap" phenomenon _can_ be decidedly racist, such as suggesting that it is the natural state of affairs and one race is intrinsically better than the other. But most educators don't claim that; instead, most claim that outside factors are artificially depressing the performance of one race, and these factors need to be identified and mitigated. The natural state of affairs is racial equality in academic performance, and it is the educators as well as the educated who need to ensure this comes to pass. I don't see anything racist about that.
Just because race is being used to organize large numbers of people (just like age, sex, and religion are as well) doesn't make the study inherently racist. It's when this separation becomes the causal theory rather than the observational organization that it becomes racist, and that has not happened with "achievement gap."