Well, SAT's are a test of what you've learned through school, so logically...
They really aren't though. SAT's are a measure of how well you can study for a test (even if that test is on things you were supposed to learn in school). I guess that's worth something too (can you do something that you don't want to do?) but if you're looking for a way to judge legitimate knowledge, you'd better look for something else.
For about $3,000 and 6-8 hours a week for a month, I could bump your kids' score on a few tests up roughly 100-200 points, depending on how low they started and how much they wanted it. For about $300 and 3 hours or so every day, your kid could do the same him/herself. It's easier if you're rich, but not impossible if you're poor.
I think a problem we have is we make school more about chasing a grade or percentage or score more than we make it about getting an education. If your kid is bright enough and determined enough, and has an idea of what is going to be on the final, he can skip the entire semester and walk in and ace the final. He's also going to miss out on a lot by doing it.
Just think back to your own college. Did you learn more by studying for a test, or researching and writing a paper, or preparing a debate?