That puzzled me at first too.First time trying the game. So can you only use one teacher at a time?
I'm not sure how the grading system works. I feel like I have a lot of teachers, but I can only use 1 at a time??
Initially you can only have one teacher. One of the teachers you can recruit opens up research, which includes options to increase number of teachers one by one up to total of 5, among other things.
Each girl learns different subjects at different pace, with each subject contributing to the grade of its own grouping (Social, Sciences, Physical) so to get higher grades you'll need to switch subjects up a bit... Or just wait a bit longer. Currently I've not seen any rush to increase grade nor any degrade in grades, with higher grades only unlocking some stuff later on in the game.