Isekai-gaeri no Yuusha Sensei no Musoutan: Oshiegotachi ga Bakemono ya Uchuujin ya Nazo no Soshiki to Tatakatteru Ken

Aiba Hashiru, after battling the Demon Lord in a foreign world, wakes up back in modern Japan with his hero powers intact. He takes a teaching job at a private girls’ high school, hoping for a quiet life. However, he discovers that one of his students is fighting a strange monster in the mountains. The school itself is linked to a "hidden world," and the students are involved in secret conflicts. After rescuing a student, Hashiru gets drawn into these hidden incidents. His immense power, once formidable in another realm, continues to be extremely strong in the modern era.