I was a long-hair male teenager in Texas and got to experience this first-hand. Besides the frequent disparraging comments from teachers and staff, I was also kicked off the track/CC team for my hair because I "Didn't match the image the school wanted to present at athletic events." I had a 4.0GPA, was active in school activities, enrolled in all AP/Pre-AP classes, and was, most importantly, good at and enjoyed running. As a freshman I ran a 5:20 mile, 12:10 two mile, and <20min 5K and was up for varsity consideration in my sophomore year. Despite this, the coach told me, point-blank, that I could only stay on the team if I cut my hair above the ear.
My parents, pissed, yelled at every school admin they could get a meeting with to no avail. Ultimately, even the principle was impotent, apologizing for how this must be "upsetting" but saying that she couldn't do anything. Apparently the athletics coordinator who made the rule didn't report to the principle, but to the district athletics office. My parents told me they would be behind me to fight it up the chain, but I decided that the experience had ruined competetive running for me and moved on.
The enforcement of white, christian, heteronormative values to teens' hair is so insideous. It is used for racism against black teens with braids, homophobia/transphobia against queer teens who don't conform with gender stereotypes, and in my case, just to be fascist assholes to a white cis-het teen boy with long hair. Nowadays I am covered in tattoos, oscillate between long/short/natural/neon hair, and have never felt like a better representative of my institution. I am about to get my PhD, was the president of my department's graduate student association, have taught and ran summer and afterschool science programs for under-represented kids, and fought for (and gotten) better compensation for graduate employees at my school.
Fuck every petty school admin who supports this shit, I am proud of my image, I am proud of teenage me for holding onto his individuality, and I hope that any teenagers in a similar situation can feel proud of themselves too, regardless of how they express.