, never heard of this thing called, "discipline"? You join a community, a club, a school, an organisation, anything; each have their own rules, you don't like it, leave it. What's the problem
In all the groups, community organisations, clubs etc that I take party in there is room for people to politely ask for a review when things seem outdated, unnecessarily restrictive etc.
Some schools would do better to behave like that.
Most sensible people respect rules which have a clear purpose and wider benefit for all. Petty unnecessary rules without clear purpose cause respect to be undermined.
There is no logic for an arbitrary 4Cm difference for the length of hair allowed for boys and girls. Is the OP not even allowed to bring this up politely with the school?
They are schools, not dictatorships.