Maho☆Gaku ~Mahoroba Kokusai Bouei Daigaku Fuzoku Gakuen~

Main character graduated from Maho Gaku University, worked as UN lieutenant, and currently is a special agent for Meguin Hunters who track aliens that live assuming form of humans.
For some reason protagonist is invited as a temporary lecturer to his alma mater. There he leads a sweet, fun and naughty life with five beautiful girls.

[From vndbreview]