I guess the best advice I can give for staying focused is to just be good to yourself. Treat yourself to something that makes you feel good. If you need to eat an entire box of cookies, then do it. Just don't make a habit of it
If you can just remember that in a few days, you will have time to take it all in and truly grieve and experience it, it will help. Don't feel like you have to do it all. Each day, set a small goal, whether its studying all your SAT vocabulary or something, and try to reach it. And then reward yourself.
Make sure you're eating well and getting enough sleep. Those two things can help you maintain a shred of sanity.
I know its hard to keep your mind from wandering and from being sad, but I guess this is a bit of a mind over matter situation.
If you can't attend the funeral, why not have a small memorial service of your own? Set aside an hour or so, and have your family talk about him.
Don't worry about forgetting him. You won't. My dad will be gone 7 years in June and I can still feel his hugs, hear his voice, I still remember his smell. Your grandfather will be forever alive in your heart. Yes, it sounds cheezy, but its true.
Good luck, girl. We're all pulling for you!