For such a risqué series I can’t help but be impressed by the quality of the character design and art style.
There’s hentai, there’s manga, there’s hentai manga, and then there’s manga like Koukousei Prowler, which is technically an H-manga but gets so explicit that you’d be forgiven for thinking it’s outright hentai. (It’s listed as “Borderline H” at Baka-Updates, which I think is a fair designation since it has a manga format).
Mangaka Isutoshi (most famous for his other H-manga Slut Girl) has a way of mixing weird adventures with random acts of sex.
Koukousei Prowler (complete)