Thanks! :)
English is not my first language, so I get stuff wrong from time to time. These small mistakes should get fixed eventually with proofreading and similar help.
Maybe there will be actions that don't advance the time of day, so you could then perform other actions with it. As for your other questions:
I'm not sure if that's a question or not but I'll try to answer it :P
The game has 4 Main Girls, and some other characters that are important to the story. You will be able to have a deeper relationship with them.
The game has lots of Side Characters. There will, at some point, exist sexual interactions with most if not all of them, but most likely the MC won't have a deeper relationship with these side characters.
Currently, the easiest way for her to get lower than 100 on the test is in the first weekly test event, where you can give her a 0 for cheating. She really cares about her grades, so that should be enough, maybe some affection/corruption can help.
There's also a very rare event where she can miss the exam, but that's hard to get.