My daughter...

She's beautiful and I actually like her hair.  Kudos to you for being a "cool" step-mom and allowing her some self-expressive freedoms.  If all else is good in her life (i.e. grades, work, attitude), then I say let her hair be a non-issue.  Teachers are so used to seeing different forms of self-expression, that I highly doubt they would even be phased by her hair at all.  My step-niece is a far different story with multiple piercings and the goth look, with a depressive and anti-social attitude to match.  In her case, the grades have never been up-to-par, she doesn't maintain any healthy relationships (male or female) and is very sullen and withdrawn.  Here's a case where she could really use someone to step in and be concerned.  The more she rebels for attention, the more her mom writes her off as a "lost cause".  Looooong story short, you are loving and caring enough to worry about her and as long as you tell her you care and are always willing to listen, she'll know it and remember should her life become challenging.   

Hugs,