Wow, it sounds as though you have tried everything, right down to not answering the door. That's a big hint especially if the lights are on and the cars are in the driveway!
You may need to have a family sit-down with the grandparents, mom, dad, aunts and uncles and explain that while you love them and love visiting with them, dropping in unannounced is causing a lot of problems for each of you. You could then list everything from your homework, mom or dad's homework from the office or school, sleep, same for your aunts and uncles. You don't really want to touch on the fact that you only want time alone with your parents because that might be stepping over the manners line with the grandparents.
In previous generations it was very common to have a family dinner once or twice per month (sometimes weekly dinners!) and it allowed the entire family to get together for a big meal - usually on Sunday afternoons. It was a tradition with many families. Talk to your parents about this and they could talk to the aunts and uncles. It would mean about two hours for eating and socializing once or twice a month. Hosting the dinner could rotate between households - your parents, then the grandparents, then one aunt/uncle, then another, etc. Another idea would be to invite them over for dinner or out for dessert or ice cream. It's keeping them connected that really counts.
The grandparents are probably lonely and this is their way of keeping in touch with you. Even though they can use email, they are of a generation where nothing beats face-to-face contact
Oh man, I can definitely relate to this. I absolutely abhor people who "drop in". I mean, what gives them the right to think they can just stop over any time they feel like it? Arrghhh! Sorry, pet peeve of mine!
Anyway, it may be time to be frank with them. You don't have to be nasty, but you need to tell them in no uncertain terms that you don't appreciate what they're doing. After all, it's your home, and you're entitled to your privacy. Tell them that you very much enjoy their company, but would prefer them to call before they come over. The fact that your aunt told them to call indicates that they still don't get it. Apparently, it's time to lay down the law, and feelings be damned.
I know you don't want hurt feelings, but apparently they aren't taking your feelings into account. Sometimes you just have to stand up for yourself, especially when others are being insensitive.
See, and I on the other hand wish I'd had the opportunity to meet ANY of my grandparents- all dead before I was born.
Be kind, and do feed your grandparents- geez.
Perhaps if you went to visit them regularly then they would not need to drop by your place instead, they are probably just lonely and want a bit of company other than each other, if you visit them in their home more often, they may not come round yours as much....
If they are coming by unannounced and it is inconvenient the only way to stop them is not to answer the door. Do not open the door, tell them it's a very bad time and to please call first to make sure you're available.