Itsuka no Haru

Itsuka no Haru manga summary: Ichihara Yui is a 15 years old high school girl. On the first day, she meets Itsuka Daisuke and mistakes him for a teacher. To her surprise, they meet again in the classroom but he is in fact a high schooler too. Indeed, Itsuka is 25 years old and he wants to enjoy the high school life he has never been able to get ten years ago.