Mrs. Calpakis was the teacher of 21-B before trying to skate on Freddy's skateboard and getting a hip injury. She was replaced by Mr. Finn as her substitute. When she got back, Mr. Finn was told to leave, but then he got assigned as a kindergarten teacher.
He was overwhelmed by the naughtiness of the kindergarteners, but Mrs. Calpakis got them under control in no time. The kids of 21-B and the kindergarteners made a deal to switch teachers in order to get Mr. Finn back as their teacher. Mrs. Calpakis is now the teacher of a kindergarten class, and is doing fine.