There was a lot in my comment you just slid right over to only address the point you agreed with, I was hoping you might address literally anything else I wrote, but oh well. We agree that it can be a form of sexism, but it is more complex than that, hence why it is an intersectional issue. Why do you believe it necessarily stops having a racial connotation just because it can be used to hurt white people?
Just because white people have been subject to abuse due to their hair length, it doesn't absolve the racial connotations and racist historical context when applied to non-white people with long hair. If this case was about an Indigenous American student, with religious reasons to wear long hair, would you be making the argument that this isn't racial discrimination? What about a Sikh student? A Rastafarian?
This is an intersectional issue, and as such, requires a little more nuance in diagnosing than "Well I don't see any white boys getting away with it, so it can't be racist!" When rules are made, they need to be evaluated on their ability to hurt people. If the rule can disproportionately hurt people based on racial elements, that rule is racist. This kid is black, part of him expressing his blackness is his hair being long, so any rule forcing him to change his hair is racist. If it was an Indiginous kid, the rule would still be racist. If it were a white kid, the rule would still be racist. The rule and the people enforcing it are racist, even if they never apply it to anyone.