NYgymfan,
I'm sorry to hear such pressure is being put on you at this age. The choice of school is far less important than how you perform once you get in. It also depends a lot on your personality and on what you want to pursue in your life. For instance, some people like very small schools where they can get a lot of personal attention from professors, they can get to know most of the students, and they can excel at an extra-curricular activity that they might not get to pursue at a larger school. Others like the variety of a large state school and enjoy the cheap tuition and the anonymity. For some a religious school is wonderful; others want a school that offers something unique like a Great Books program or an art program. No school is the top at everything. And the amount of tuition does not relate directly to the quality of the education -- fads occur in education as well as merchandise, ($200 tennis shoes are not necessarily better made than $30 shoes, and $40,000 tuition doesn't mean you'll get a better education than $15,000 tuition. It depends on what you want). The cache of a name brand school can help, but graduating in the bottom half of the class at Harvard is not superior to finishing at the top of your class at the University of Tennessee, for instance.
As to scores on the tests, both are normed -- which means the score reported refers to how well you scored compared to the population of test takers. For instance, the mean (average) of both tests refers to scoring at the 50th percentile (1/2 of test takers did better, 1/2 did worse since supposedly the mean and median will be the same). On the SAT's, the mean is 500 on each scale, with a standard deviation of 100. On the ACT, the mean is 20 with a standard deviation of 5. Relative performance on each can be compared. General markers are as follows:
_____________ACT_ 5____ 10_____15____20_____25____30_____35
_____________SAT_200___300____400___500____600___700____800
_________________.1_____ 2%____16%__50%____84%__98%___99.9%
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Anything over 500 on each scale will get you into a decent school somewhere. Anything over 600 on each will get you recruited at some schools. Get 700 on any scales and you will be very desired at good schools. But the most elite schools get so many applicants that unless you REALLY stand out somehow, or know someone, you are very unlikely to get accepted. I've known students with high 700's or even 800 on SAT scales that didn't get accepted at Harvard, Princeton, MIT or Stanford -- not because they weren't bright enough, but because there are other considerations. The schools want balance...
The best advice is to pick about 9 schools you'd be happy attending - 1 or 2 dream places with little hope, 4 or 5 very good places with reasonable hope, and 1 or 2 good places that you're fairly sure you can get into, and apply to them all. Apply EARLY, visit the campus, and make an impression. And remember that your life isn't ending with this choice, it's only just beginning. You can go anywhere from anywhere anymore. Pursue your dreams and be flexible for detours.