So he's told you that there is a possibility that you can stretch your ears if you get A's and B's and already you're saying, no, that's too hard, but let me do what I want to do anyway? That's not very fair and that's VERY immature and stupid on your part to just give up on your grades and assume that you can't do it and get rewarded for that, and this is coming from someone that has 0g, multiple piercings, and tattoos.
He's made it clear that you have to work for it, and as your legal guardian, he has every right to dictate your appearance until a certain point. Stretching is not as easy as people claim it is and it costs MONEY. Where are you going to get that money? Probably from HIM, huh? You need to buy the tapers and the plugs/eyelets and let them heal for 1-2 months the bigger you go. This isn't going to be overnight; it can be a long process depending on how long it takes you to heal. He's going to have to cough up the money to allow this, so you think you can just talk him into it and have him spend the money on you too, and you DON'T have to work for it?
I think it's HIS MONEY and he has every right to make you work for it and say no if he wants. Your attitude does stink, even for a teen.