Oh, God.
'Ruby, now 18, claims the school's head teacher Richard Brown told her that her hair was "too big".'
Or, in other words, the rest of the staff were quietly dying inside at the prospect of a wealthy white bloke ripping into a teenage girl for having hair characteristic of her ethnicity.
As long as it isn't full of nits or filthy dirty, there is absolutely no reason to comment upon any girl's hair other than to say it looks great. In those first two cases, it's relevant for safeguarding/potential neglect, with the latter, they spend enough time insecure about their appearance and being criticised for whatever they do/look like that actually saying something nice to them is all too rare for many.
Rules about make up and nails, along with particular rules about health and safety (such as in DT), fine. They affect girls of all ethnicities equally. But not hair. Not anybody's hair. It's part of them and I would no more say they have to make it straight, put tons of expensive product on, straighten or cut it than I would tell them they have to lose weight because their breasts or backside looks too big in their uniform.