Yamada-kun and the Seven Witches S01 E01 I've Turned Into Her!

Yamada Ryu, a second year student who attends the prestigious Suzaku Private High School, is a problem child who comes to school late and leaves early, is prone to fighting all over the place, and even nods off in class. One day, he falls down the stairs with the honor student Shiraishi Urara. They faint, and when he wakes up in the infirmary he's turned into her!