Enkou Shoujo: Rikujoubu Yukki no Baai The Animation

Slutty gyarus, you like them, I love them, everyone should appreciate them and I've got two animes perfect for the job, both are set in school, there's sluttines, two or more girls in both, Enkou Shoujo is seen from the perspective of the girl turning slutty and doing with various guys, JK Bitch has only one guy on the job to all the girls, and to top it all both came out the same month and almost the same day, so both are great. 

Both involve slutty highschool girls pleasing another students or old salary men, both with some gyaru love thrown in there too (Energy Kyouka more than the other, definitely), Enkou Shoujo Shows the process of the female becoming a slut, which is actually more creative in execution, but both are great. 

Both hentai are based on games by Frill illustrated by Koizumi Amane with focus on innocent high school girls becoming prostitutes.