Sir is in love (先生氏が恋をしたの巻) is episode 360 of the 1981 Ninja Hattori-kun anime.
Mr. Koike is smitten when a young female teacher named Aiko becomes part of the staff and the children take the mickey out of him. Fortunately, Kenichi realises his mistake and gets Hattori to get his disgruntled teacher out of his depression of love mishaps.
The headmaster introduces a new teacher to the school - Miss Aiko. She greets the students, who reciprocate chirpily, but Mr Koike seems more interested in her than anyone else - in other words, he's in love with her!
Later on, they go to her class for music lessons. The door being slightly open, Mr Koike stares at her, unable to comprehend his love for her. He turns away and talks to himself, saying that he's undoubtedly in love with her.
During geometry, Mr Koike draws a love sign on the board and gets laughed at by the students. Instead of punishing them, he desperately tries to cover up his mistake. Then, during PE, he falls over a ramp while demonstrating the high jump when he sees Miss Aiko with some of the younger children.
Then during breaktime, Kenichi, Yumeko, Kemumaki and their classmates jump to conclusions, correctly guessing that Mr Koike is in love with Miss Aiko. Kemumaki then devises a nasty scheme...
Miss Aiko is playing her piano when Kemumaki comes into her classroom. He lies to her that Mr Koike is a fan of Mozart, but he can't ask her. Miss Aiko promises him that she'll ask him if it's true, and Kemumaki tells her that Mr Koike is in the playground.
Miss Aiko strolls out into the playground and sees Mr Koike sitting in the shade of a tree. He is shocked when he sees her, but manages to talk to her; however, before she says whom she likes, he assumes that she feels the same about him. The kids laugh on.
Later on, Mr Koike is passing the lab, when he sees her; however, it turns out that it's a heavily disguised skeleton. Slipping on a bin, he rushes out of the lab.
Back at home, Kenichi tells Hattori about what he did. Hattori is disdainful and says that Mr Koike is serious. Kenichi feels guilty and asks Hattori to do something, to which he agrees.
That evening, a distraught Mr Koike is walking home when he bumps into Miss Aiko. She tells him that she likes his compassion for his students, and Mr Koike is overjoyed. Once out of Mr Koike's sight, however, Hattori takes off his disguise and he and Kenichi rejoice.