I'm pretty certain that until you do GCSEs (or whatever you do in America at 16) most grades before that don't mean a lot. I'm sure an A at 11 is very different to an A at 16. When I was 11 I was getting As and Bs, but at 16 I got Bs and Cs. Did I get more stupid? no (well I hope not) Things just get harder. Some people have the intelligence to do it, some don't.
If you wanna go to uni get off your bum and start studying, or at least stop moaning about the lower grades and do something about it
*nods*