Shishunki no Obenkyou (Series)

Puberty is a time when many become interested in the opposite sex for the first time, but Kasuga is hit more than most. She becomes so obsessed with thoughts of penises and balls that her grades are affected. Thus, when her childhood friend, Kobayashi, bets her that he will have a higher score on the next test, with the winner being able to request anything from the other, she sees her opportunity to witness male genitalia in the real world.