They dyed their hair brown.
Every single one of them! Even the girls who had great hair! Most, if not all, high schools here ban hair coloring or ostentatious style displays in favor of uniformity and group harmony. But colored hair is the fashion in Japan, with millions of people- male and female- sporting brown or auburn locks these days. So I guess dyeing your hair is now an official rite of passage in Nihon.
It's quite a change after seeing these students with black hair for almost a year. Each looks very different now. We do have one girl with naturally brown hair who is constantly in trouble with teachers and school officials who insist she must be dyeing it. Think again...
On the other hand, from what I understand, there's a growing move towards natural hair. All it would take is a popular idol to give an interview extolling "keeping it real" and the pendulum would swing. Hamasaki Ayumi, I'm looking at you!