What a crock of shit! Another reason I'm glad I moved to Australia. Almost every school here has strict, can only be purchased from the school uniform shop uniform. Thankfully they don't have arbitrary rules regarding personal appearance.
At my eldest sons academically selective school, where last year, 100% of the students who applied to university got their first choice of both uni and course, the students can be seen sporting long hair, super short hair, graduated cuts, all colours (dyed) and styles, facial piercings etc. The school ranks 13th highest in the state, and is the only state run school in the top 20, the rest are fee paying.
So I absolutely disagree that schools with hairstyle rules perform better.
Fwiw, although I do like uniform, there is not a chance that it prevents bullying based on poverty. The poor kids who end up with uniform from the school second hand store are all faded, marked, tattered. Their 'white' socks are grey and their black generic shoes are scuffed through at the toe. Yes, they are wearing identical clothes to everyone else, just about five years older, usually worn by at least two other people and often not clean.
OP, I don't see why your son should spend time in isolation when their stupid rule wasn't even properly specified.