Actually they do. But not like you think. Rarely does someone do crappy in elementary school but excellent in HS. So, it is an indication that unless you start to kick butt in elementary school, you aren't going to do that well in HS. SO study!
No, not at all. Colleges don't pay any attention to your grades until 9th grade. And that's only if you're a major athlete or have won tons if awards and stuff like that. But keep a record of goods grades for things like recommendations