I assume you're playing on Normal difficulty, because on Easy it lets you skip classes without any problems. If you've already completed Chapter 3, you can go to the rooftop in the mornings before the first class and talk to Hi to skip classes with her.
And no, I'm afraid that neither showering nor eating with the girls helps improve your relationship with them. The only things that actually do are the personal quests. Everything else is just extra content to get more scenes with them. Although some choices can lock you out of certain routes. For this game, I wanted to avoid having a points system so that things would feel less mechanical.
And the idea is, of course, to add more content to the game so that you have more things to do during the week and don't get bored while waiting for Sunday, since it's a slow-paced game designed to last quite a while. But of course, for now I suppose there simply isn't enough content. I apologize for that. I'll keep improving it with future updates.