but school does not exist to provide avenues of personal expression.
Nor does it exist to prohibit avenues of personal expression. Man, what the heck happened to you guys to make you so apeshit for rules? Consider:
The Boston Tea Party
I regret that I have but one life to give for my country
The Underground Railroad
Being the first country to not bow our flag to the host country's flag at the Olympics
It is resoundingly American to be defiant and rebellious. This girl simply has pink hair. It's not a revolutionary action. It's not going to lead to chaos or anarchy. Dogs and cats aren't suddenly going to get along.
Personally, if this girl were my child, I would be as proud as I could be.