Based on

Movies like Hell Teacher Nube: At 0 a.m. Nube is Dead!! (1997)

Nube gets a new student called Jun Kagami, a shy boy who has just transferred to Doumori Elementary School. Due to his insecurities, Jun believes that his only friend is a mysterious clown entity that lives inside mirrors. It is up to Nube to find out the true nature of this clown.