Anime: Chou Kuse ni Narisou

It's about Shiratori/Ootori Nagisa, a martial artist/idol singer. Just for laughs she decides she wants to go to a public junior high school. In order to remain incognito she hacks off her pink hair, dresses in a boy's uniform and assumes a false last name. She joins a gang at school and falls in love with a high school boy, Nosaka Akira, who is a martial artist himself. Nagisa is a very cheerful character, and very handy in a fight. Between her boyfriend, her career and her dad's dojo she has way too much to do; she usually sleeps through class, unless the class is rioting or ninjas come to the school to attack one of her classmates or robbers attack the school and hold the class hostage or she is being harassed by a music instructor with a crush on her who thinks Nagisa is a female impersonator...

Source: AnimeNfo