The entire reason for the existence of private schools is because some people are willing to pay money to put their children in a more controlled environment. The type of environment where they will (hopefully) get a better education, and stay closer to the values of the religion that the school is based upon (since most private schools are religious in nature).
The controlled environment that these parents are paying their money for generally does not include blue hair or any sort of student "originality" or "rebellion". The facts are this, most parents who send their children to private school do not want their children to have blue hair. Nor do they want their children to associate with people who have blue hair. These people are not paying their money because the private schools have better teachers (they usually don't), or better facilities (ditto). And they certainly aren't paying thousands of dollars so their children can go to school with "common street trash" (which is what any goth, punk, raver, or anyone with blue hair is going to be in the eyes of your average private school parent). They are paying their money for strict discipline in accordance to the rules of their given religion.
Now I am aware that there are no religions that expressly state "Thou shalt not have blue hair". But, it is one of those things that can be argued (or implied) from the religious works of at least the Judeo-Christian religions. This is something that will likely not change for a long time, if ever. Consider public school if you disagree, because private school and blue hair just don't mix.
A note to all you Britnoders out there.
In
America, private schools are the ones you pay money for, and public schools are the ones that everyone else goes to (
state schools).