I think the Classroom and Courthouse areas haven't been fully debugged yet, or perhaps they're simply not fully implemented. Changer has a lot on his plate, especially with COVID-19 still running rampant. They are supposed to increase the selected girls' levels over time, I believe. However, my girls are all absurdly high levels, so it might just take too long for me to notice a change.
Edit: Function confirmed. The higher your Loyalty level in Campus and City, the faster the girls studying in the Classroom and Courthouse level up. However, higher level girls will gain levels slower than lower level girls. The progress of a level they're studying for is measured in 0-100%.