Kusatta kyoshi no hoteishiki (1994)

Atsushi Arisawa goes to Jougaouka High School so he can be closer to Masami Shibata, a boy who was his childhood crush. But Misami is n longer working at the school and his brother Masayoshi is the nurse now. Atsushi accidentally thinks that Masayoshi is Masami leaving him horrified because he thinks Masami's personality has changed.