Wataridori no Mon wa Tooku

Petroshkim Gymnajia is an elite boys boarding school for society's future leaders.
13-year-old Lodis and 14-year-old Yurius meet a young boy as the summer vacation comes to a close.
The boy is their age but works as a teacher. His name is Jil.

The three quarrel and become good friends, until one day Jil vanishes.
Lodis and Yurius need to know what happened......

[From DLsite English]