dude.. just the fact that you got an A is AWESOME!!!! i would have gotten an ice cream for that....(a+ is fro-yo) but A GROUNDING?!?!?!?!?! im sorry but your parents are messed. if you want you can ask your teacher to retake the test and get a better grade.
I'm very sorry you have such unrealistic parents. I think it's great that you got an A. If you feel you gave your best then you should feel good and not stressed. I'm sure your parents think that being tough is going to help you. They want you to push yourself. Did they have A+s in school? The best opportunities go to the best students and they want it to be you. Don't make yourself a wreck over it. If you can do better then do it. When you graduate you won't have to deal with it anymore. Are you parents open to a conversation?
Your parents sound like overachiever whack-jobs: they need therapy and some tips on parenting. Keep pulling A's and you'll be looking at merit scholarships in your future - where you can get out of your prison and live on campus.