Kusatta Kyoushi no Houteishiki

Arisawa Atsushi enrolls at Jougaouka High School to be near Shibata Masami (Ma-chan) who was his childhood crush. Unbeknownst to Atsushi, however, Masami no longer works at the school and his brother, Masayoshi, is now the nurse. Atsushi mistakes Masayoshi for Masami and is horrified because to him, it appears that Masami's personality has done a complete 180. Add to the fact that Atsushi's friend, Inagaki Kouji, is totally in love and constantly trying to make out with him, Atsushi's life at Jougaoka is hell—until he starts thinking that maybe the new Ma-chan isn't so bad after all, and that maybe he actually likes the way Ma-chan is now. Until, of course, he finds out that Masayoshi isn't Masami. - AnimeNewsNetwork