You'll soon unlock a few more "school" minigames, and the only stat that matters is your highest grade in any one game - so there's no reason to even try to be good at all of them. Just pick one you like. (And if you don't like any of them, one is a memory game that you can quickly "cheat" with a pencil and paper, or even just a screenshot!)
In addition, these games don't speed up progress meaningfully at all - it's really just a quality of life boost. (It lets you fast-forward a single mechanic for a few minutes at a time, which is nice to make a prestige less tedious, or to buy an upgrade you can *almost* afford just before bed.)
I don't know what you mean by "100% the game," as there appear to be an infinite number of levels for each mini-game, so you can never "100%" any of them no matter how many times you do them. There is an achievement tied your highest grade achieved, but you only have to do that once in the game of your choice. (And I believe it's a hidden achievement, so it's not even part of the game's "real" progress and doesn't give you any completion bonus.)