Baka Dakedo Chinchin Shaburu no Dake wa Jouzu na Chii-chan

Based on the erotic game by OrcSoft.

One day near the end of summer vacation, Kouta runs into his classmate Chieri near his home, where he lives alone. He was waiting here because he wanted to learn from Kouta, an honor student. Kouta agrees and teaches her how to study, and she offers to do something naughty with him in return, albeit with no actual intercourse since she has a boyfriend. Kouta becomes more and more attracted to Chieri's surprisingly honest and dedicated personality, who says that she will take care of his dick instead of studying...

(Source: Otona Bunko, translated)