PeroPero☆Teacher

The erotic game by Cosmos was released after the release of these 3 OVAs.

Milky-chan is tasked with caring for her shut-in male friend while his mother is out of town. Feeling she's the only one who really understands him anyway, she accepts the job happily. When he won't come down for lunch, though, she goes up to his room and catches him masturbating to some hentai. Then she finds a book about having sex with women, so she volunteers herself so he can learn and experience the real thing – but it's not complete until she puts on a costume!