I don't remember my SAT score, but my ACT was 29 and I'm fairly sure that's at least in the middle 50% of the U of I.
And now you see the difference between the ability to test well and to write well. I could always outscore my classmates on SAT/ACTs because I simply knew how to do well on multiple choice tests (to the point of being the only National Merit Scholar in a 600-member class), but my writing skills were (are) not on the same level. College admissions are much more difficult these days - test scores are only a starting point, not the end-all, be-all they were decades ago. Your essay is your chance to say "hey, look at me, not these other bozos with similar scores."
You're getting some good advice, even if some of it seems harsh. As cornbread says, work the essay with your English teacher and/or (if you're lucky enough to have one) school counselor. The page ploto linked says:We want to read about you, what interests you and why. After the essays are completed, ask a teacher, counselor, or parent to proofread and provide feedback. Remember, they should not write for you, but help catch misspellings and give suggestions.
So, to address another part of your original question, you're not writing about why you want to go to IU - you're writing about the things that excite you and what you want to do in life. If you know that IU has a great program in your field of study, you can work that in, but as a part of a greater whole.
That's my $0.02. Good luck!