This is normally not advised as anything you hide from your parents will almost always get found out. Especially if it is important. However, sometimes drastic measures are needed.

[edit] Steps

Monitor your home mailbox around the time you expect your report card to arrive in the mail. When you see the report card, remove it from the mail.
Send the receipt return back. If your school requires a signed copy of report card returned to them, you will need to forge your parent's signature and return the copy. Be warned that this might be illegal and is certainly a very bad idea. When you are discovered you will have disciplinary action from your school and your parents to answer to.
Try to stop any conversations leading into the dangerous zone of grades. These can very quickly get suspicious with other parents or even between you and your parents. Realistically, you should expect that your parents will be persistent enough to keep asking till they find out what your grades are.
Don't bring up the topic of school because this will automatically trigger a discussion about how weird it is that your report card hasn't arrived.
Destroy the evidence. If there is no need to keep the grade on this planet then bury it in a corner of the lot where it can safely decompose. You could also try burning it.

[edit] Tips

If you get caught just flip the script. Say how hard it is to do well when they put all this pressure on you to be the best student when you're not. Even cry to get extra points they may be mad at first but they'll give up after a while.

[edit] Warnings

If you get caught your punishment will be ten times worse than it was going to be before. Forging a signature is a serious betrayal of trust as well as being potentially illegal.