I was thinking Bella was going to fill in because DPC made a point of having MC and Bella talk about her being his teacher in the spring (and it's an entry in the Bio). Since the game is unlikely to time skip that far, this would be the perfect chance to explore those fun, awkward teacher/student interactions. You could have Bella publicly rebuff MC in class only to be passionate with him as soon as the other students file out, leaving MC conflicted. Any maybe KRJ (is she in MC's English and Math classes?) comes back into the classroom because she forgot her ear dildo or something and sees the two hot and heavy. Will she blackmail them? Nah, probably not because everyone on campus must know about MC and Bella by now since they make a poor effort to hide it.
However, I've seen just as many plausible or more likely scenarios posed that have a brand new teacher coming in instead of Bella. Maybe Bella could surprise substitute for a day, which would tackle both routes. Ideally, I'd like to see a new Math teacher, have Bella for English, and somehow keep Cathy involved in the story. I don't want to see her gone completely. DPC will probably make her drive off of a cliff though.