Nee Sensei, Kocchi Muite

That was his first kiss.
Keito, a high school teacher, learned of his student Yui's feelings after receiving an unexpected kiss. At first, he only saw Yui as a student, thinking he was just mistaking his admiration for adults for love.

But after witnessing Yui's feelings, which were more serious and earnest than he had imagined, Keito wanted to confront those feelings. He was conflicted about falling in love with a student, but he found Yui's friendly and honest nature adorable, and before he knew it, he couldn't take his eyes off him.
[via chill chill]