Pactio 31 ~Class Zen'in Boku no Geboku~

One day, a mysterious magician encircled the academy with a magical barrier.
In response to this emergency, the dean orders you to make pacts with all of your students.
Under the graceful leaves of the great world tree, you ought to have sex with
them in the guise of contracts with your students.

Can you make pacts with all of them and discover the one who threatens the academy!?

[From DLsite English]