I'm Not Afraid of Onions!(1990)

A girl (Kiyora Morinouchi) on the way to school literally runs into a scary-looking man with a scar and sunglasses (Junichiro Yaguchi) who drops a butcher knife when she knocks him down. She immediately thinks he's a member of the local yakuza.

After quickly collecting her things, Kiyora heads to school only to find out the scary man she ran into is not only her homeroom teacher, but her home economics/cooking teacher as well (hence the butcher knife). She's not a very good cook, so her Yaguchi-sensei assigns her to make a bento (box lunch) for him every day until she gets it right.