Hippies do have the right to grow their hair as long as they want. No person who thinks it is appropriate for the government to dictate citizens' hair length is anything remotely like a conservative. They are instead leftist totalitarians who have made the state their god.
I'd think most people would describe the Amish etc as pretty conservative in their beliefs and how they live. The Amish wouldn't put up with somebody outside their society telling them how they should dress or wear their hair, but they'd certainly consider it appropriate for the society itself to decide if something was crossing the bounds of what is acceptable.
If people in a community elect school board members that set those hair-length rules, and the community (with only one known exception) is happy with those rules, then they should be followed.
Do you feel that it is opposed to conservative principles to limit people to only marrying one person at a time, or of the opposite sex, or even that they be limited to marrying people, as opposed to somebody marrying his dog? Thats a personal liberty issue.
A society can define the rules to differentiate the people that wish to be part of a society apart from those that don't.