An easy way to max out stats quickly with the girls is to start off with bath skill, then max out your fishing skill and buy out as much lure as possible. Spending your time fishing is required for the yellow back-pigtail's girl's study meter alongside bugs, as well as having a chance to give you present boxes. These present boxes can either give you 20+ affection on the pink or max out the blue study bar once you gift it to them. Fishing also gives you a small amount of points used to gain skills, so it's a good way to grind for skill points.
You can do the dish-washing mini-game for easy money, as long you don't mess up the perfect bonus is pretty good. Buying sodas or the item that increases the white sperm counter for sexy time is another good way to quickly gain skill points. If you ever have shit RNG, just save scum each time, pressing F5 restarts the game for quick access to the main menu.
This game has a fair amount of grind, I suggest you do that first before you fap, otherwise you might lose your boner before the next big scene.