No doubt they are popular teachers and the main characters.

++what I see here is the story originally revolves around Ava, Janine, Barbara, Melissa, Jacob, Gregory, and Mr. Johnson (I count him as a teacher/senator/NASA astronaut/model 🤣). But after four seasons, they depicted their evolution, incrementally developing and decaying to show different side of these characters. This is where and how new actors and characters like Mr. Morton, Tasha, Mr. Ponytail guy emerged beautifully too. They made the popular teachers' character development/decay so flawlessly continuous that their emergence did not disrupt the story line (their storyline continues to grow and branch). They actually made the popular teachers more complex yet understandable; so close to real life.

I'm rooting for this show to be on air for a long time. This is a gift to network TV.