They literally already lost a federal case on the exact same issue (with, surprise surprise, a pair of other Black students). The CROWN Act that is being referenced here was passed because of that past case. And “long hair” is a style, and one with particular relevance to some cultures (some Native Americans and in the past case the student’s family from Trinidad).
They’re not sticklers to the rules, they’re just racists echoing the racists of the past by policing and restricting non-white hair styles.