I once knew a family with a teenager who was grounded for two months any time she got one grade below an A.

She was grounded for the majority of her life because one of the teachers either had it in for her personally or was a tough grader...I was not clear which...but it wasn't an A, so it didn't matter WHY she didn't get the A.

My parents had the much healthier approach...I wasn't expected to get As in everything, but I was rewarded at the end of term commensurate to how many As I did get ;). That got my grades up, believe me! Carrot your kids, don't stick them.