Saimin Gakuen 1-nensei

The protagonist is a student of a third rate university with an awkward personality and sullen looks. His interests are deviant as well, but as an adult he has no shame about this. He acts like a stalker, and sneakily digs through the trash in the middle of the night. Naturally he's avoided by everyone around him.

At one point, he picked something like a toy gun out of the trash. Wondering if it might not have some monetary value he began to fiddle with it, and unexpectedly it became clear it was a device for inducing hypnosis.

Then, knowing it would normally be entirely impossible, using that device he became a student teacher in an all-girls school. Through this, the protagonist set about satisfying his own desires to his heart's content.

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The protagonist lays his poisonous fangs primarly on the first year students but of course the other heroines will also appear.