Ao to Shiro de Mizuiro (2001)

Uchiyama Kaede and Shiina Kasumi used to be best friends. Now in high school, Kasumi grew more popular than Kaede, which has led her to continually bully her ex-best friend. One day after school, Kaede meets Kishida Takumi, a bike thief who's especially talented in picking locks. She asks him for a favour: if he could possibly open the key to the school roof. Meanwhile, the arrival of a new teacher seems to bring changes to the school.