Magia Mystica -Seirei Majutsushi-

Jailbait protagonist, Manase Yui, works as sorta young prodigy member of the super secret magical organization Ogdoas after basically growing up there.

4 years prior to when the game takes place, betrayal of the only other person who’d developed any trust in past Ernei, Glenn, leads to Yui getting a cursed magical artifact stuck deep up his ass that forces him to forever be stuck in a shota body while dealing with his body constantly hungering for other peoples ‘magic’.

One day, Yui gets sent on a mission to deal with a series of incidents happening in an all-boys school – Seishin Academy. Thus starts Yui’s first experience of actual school life while hiding his identity, acting out his duties as a leader of the team that was gathered there, while keeping his secret stay hidden from the others; facing Glenn as the possible mastermind behind the incidents
and possibly finally pick someone as a partner for a ‘contract’ from one of his teammmates.

[edited from Koorin's review]