Mugen School is a elementary, junior, and high school. It is also base of operations for the Heart Snatchers. Doctor Tomoe was a professor there, and used the school to fund his research.
Byruit and Cyprin both stole Pure Hearts from the people taking exams there.
It was destroyed when Pharaoh 90 was summoned and began to bring the Silence about, destroying Mugen School, and leaving a crater where it was once Pharaoh 90 was destroyed.