Ao to Shiro de Mizuiro (2001)

Description:

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.