This.
As several people on this thread have already pointed out and the "Y my Alfee no have shaved head???" crowd are ignoring.
It's an aesthetic that goes with a particular, dangerous and detrimental behaviour pattern amongst young people and it has no place in schools.
If only banning the hairstyle stopped those kids from being like that.
As for the crazy colours etc, kids don't need to learn how to express themselves through hair/makeup/piercings/tattoos/where does it end? They work out how to do that all by themselves after their personality/identity has stabilised, and it's our job as adults to ensure their default appearance is socially acceptable enough for them to gain meaningful employment.
Just because specific parts of the notoriously hard to get into (or out of; they can't sack you for anything) "diversity-obsessed" public sector lets grown adults have attention-seeking pink hair doesn't mean it's in the best interests of young people at the mercy of the job market to be led to expect that they can do what they want and walk into a job at their local employers.
For example, teachers aren't generally allowed over the top hair either (despite the fact support staff are), if the children want to get the entry-level roles to start careers as teachers, doctors, lawyers or police officers, they can't have attention-seeking hair either, and we need to give all children a fair chance and realistic expectation of what their working life will be like.
Once they are established in their chosen thing, they can work out for themselves whether to push the boundaries or advocate for changes to rules in their workplace etc, but while they're young it's our job to stop them doing things that will disadvantage them.