A guy at my school who has the best mohawk I've ever seen was asked to stop wearing it up a few months ago, because "it was too tall and distracting" or whatever from what I've heard. However, several black people in the past have had no problems from the administration for having afros that are not only taller than this guy's mohawk, but also much, much wider (of course).
Oh, and the guy's friend was asked to stop spiking his hair because the spikes were "too sharp". Yes, I can so see his friend stabbing someone with spiked hair...
Anyway, plenty of other kids at school spike their hair as well, and I've never heard of someone getting in trouble over it before now.