Presumably this is a private, religious school and, apart from meeting legitimate state requirements (not federal) concerning substantive subjects, it should be a matter of choice by the parents which school a child attends and that choice can turn on matters of hairstyle if that is the parents' preoccupation.
When children are forced into a government school, we must turn ourselves into pretzels protecting the civil rights of the children and balancing those rights against the need to educate without distraction. Generally, the distraction comes from other students or their parents reacting. Often the decision goes to an unelected bureaucrat who administers the school and comes down like a fascist against a distraction so the First Amendment rights, indeed the rights contained in the bill of rights, are sacrificed to the homogenization requirements of educrats.
Under a voucher program, the students are presumably among compatible colleagues and when they find they are not they go find a school which is more congenial.
The converse of that scenario is that once the school starts accepting vouchers from the taxpayers, the school may have to obey the state's rules.
You are among the very few conservatives who “get it”. Government owned and run socialist-entitlement schools are a First Amendment and freedom of conscience abomination. Every one of the First Amendment protections against government suppression of ( speech, press, assembly, and expression of religion) are trashed in every government K-12 school in this nation. ( Yes, **every**) And....Since it is impossible to have a religiously neutral education students are forced to endure the government's sponsored religion of Marxism, socialism, and godless secular humanism.
This was a private school. Parents, teachers, school directors, and the principal have agreed upon certain standards. Just as in a restaurant, if the parent doesn't like the menu ( school standards) they can go elsewhere.
Personally, ...I ask the girl to buy some barrettes, pony tail bands, or ribbons to get her hair under control.