It's not a lot of grinding at all, actually. As I said, within less than a week and you'll have discipline up to snuff and grades won't decline. It's simply a matter of priority. Focus solely on discipline for the first week and the rest of the game will be far, far easier. The newbie mistake (one I made myself the first time) is trying to spread out focus right off the bat. That will come once discipline is over 20.
Trust me, once you've done that very easy step, you'll be doing a LOT of punishing. And it just gets better and better as you go.