I don't actually care about C's on her report card in sixth grade, but I will obviously care a lot more in ninth if she can't get it turned around by then.
Will she be allowed to take advanced/honors/gifted classes if she has Cs? I'm not worrying about DS' grades too much because I can't--but he won't be able to get into the IB program with Cs. I've known kids whose teachers refused to "approve" them for honors classes based on poor grades due to missing homework, when they clearly had the ability.
