We might have slightly differing reasons why: for me it’s more about sexism and force conformity, for you it may be more about cultural/race discrimination.

I don't like these aesthetic rules for all the reasons I initially provided, which includes your provided reasons. They are invariably a combination of sexism, queerphobia, racism, religious persecution, and are generally authoritarian in a way that only exists to hurt people.

Let's dissect the segragated school example, considering only black and white students. Pre-integration, forcing the white students to have short hair is a sexist and authoritarian rule from an explicitly sexist and racist institution. Upon desegragation:

1. The school administration is almost definitely still racist, despite being forced to educate black students.
2. There is now a new population at the school with different racial characteristics, cultural norms, and historical context.

If the school was genuinely concerned about equality for the black students, they could reevaluate the rules about hair and gauge whether or not it will have an outsized impact on the new black student population, which it would given the cultural context. Parallel to desegregation efforts was the reclamation of natural black hair among black people (afros being the most iconic example), many of whom had been forced or coerced into white-coded hair styles since slavery ended.

Counter to this, if the school wanted to hurt the new black population, they could maintain the rule and use the equal application of it as a shield against people crying foul. The rule is still sexist, as a part of an explicitly sexist institution, still authoritarian by the very nature of the rule, but the school's racism has become implicit rather than explicit given who it now has the power to harm. This has been the racist playbook example since slavery was abolished, sliding the scale towards more implicit racial strategies in a culture that is less willing to engage with explicit race discrimination.

In the midcentury, long hair among white men became a symbol of the white counterculture, so curtailing it was authoritarian and sexist. At the same time, natural long hair among black men became a symbol of both black counterculture and black empowerment/liberation, so curtailing it was authoritarian, sexist, and racist. This dynamic exists to the modern day, and applies to different minority groups than just black and white people.