First Love - Zutto Daisuki

Amamiya Shin is a high school girl with no friends, and too shy to make any. One day during lunch, she finds a boy lying in the hallway. When he wakes up, he asks her for her name. His name is Haruki, a third year, and obviously a playboy. His first approach wasn't so chaste either. Even after Shin rejects him, he continues to show up every day. He nicknames Shin "Kami-chan" because she seems to have come straight from God. Shin falls quickly for him, and one day after class they make out.

(Source: MangaUpdates)