OPPAI Succubus Academy Sucky and Busty, Demonic and Lusty!

The protagonist is a member of the occult club who endeavors to summon a succubus to satisfy his sexual desires. His cute childhood friend, Mako, makes fun of his delusions, but when he casts his lust-fueled spell... he and Mako suddenly find themselves in the bathhouse of a succubus academy in another world! In Faren'ch Deshka Academy, all the students have boobs that have grown to the brim with milk and bodies fit with perfect proportions!

Alas, it turns out our protagonist has been summoned by the academy's chairman in a summoning gacha ritual in order to help revitalize the academy, because its succubi aren't lewd enough. The buxom chairman speculates that perhaps his succubus-summoning spell and her incubus-summoning gacha were synchronized and ended up crossing streams.

Apparently, the academy is in danger of being closed down by the Lilingard Ministry of Writhing, which has declared that Faren'ch Deshka must raise its erotic rankings within a year. To save the academy and its source of ero-magical power, the protagonist will need to turn the students into extremely lewd, sexy and obscene, wet dream-inducing succubi!

Questioning his own ability, the chairman uses her "Sex Scouter" succubus skill and discovers that he has been bestowed god-level erotic skill as well as the Legendary Book of Sexy Magic! The chairman has no choice but to recognize his legendary powers and entrusts him with the nigh-impossible task of saving the academy. To turn the students into full-fledged succubi, he'll have to seduce them! And with the Book of Sexy Magic at his side... it might actually be possible to conquer this school and become invincible in this new world. It's time to rebuild the succubus school and build a succubus harem!

[Edited from Steam]