Got it. Hmmm.
Just get a run up them. How long until your grandparents take them? Hurry, and get a cheep run up fast before they come. Find a job, since you're good at taking care of chickens, I bet local poultry owners would pay you to clean out their coops. If you have any neighbors then I would go ask around, you can advertise it on FB to. You could do grand work as well. Its not much harder then poultry keeping, and pretty simple once you get the hang of it. Older folks will definitely appreciate the help as well.
The best possible way to convince your parents, would be to tell and show them you have a plan, make it sound something like this-
"I love my birds so much, and am willing to clean up my act for them. I'm going to work hard on my grades, and planning on getting a job to help pay for feed, get a run up to protect them from the dogs/keep poop at bay. Maybe even buy an incubator eventually to raise and hatch chicks. Then we can make a profit off of the poultry. Until then, I'll advertise table and hatching eggs for sale and make money off of that."
Try to find some leads before you talk to them, you'll likely get an, "I'll believe it when I see it" response from your parents. So, you have to make them see it. Show them were you've found cheep material for the run, and maybe even some local poultry owners you're going to ask about cleaning their coops. If they still don't believe you, then start doing it. Make an advertisement on Craigslist, FB, and even a stand at the end of you drive way for eggs, ask around for a job, and show your parents your willing to clean up your act, work on your grades, and take this into your own hands to keep the poultry.
I think if you really put your mind to it, this could totally work. I hope you pursue the effort of keeping them! Good luck and keep us posted, I would be so sad if I had to give away my babies.