Hitotsu No Shigusa Mo

From Aerandria Scans:
His name is Itou Hiroyuki, nickname: Hiro-kun. He’s a math teacher at the private Taihei Academy, as well as the homeroom teacher of class 3-3. He’s young, cool, and tall, and yet, when he’s with the male students, it’s impossible to tell who are the students. He’s the person I love.
This is a sad and gentle story about the 15th winter of a girl who fell in love with her homeroom teacher.