<p>Alright, even if each of your SAT Subject Tests were 100 points higher, they would still be weak… yes, including that math one, because a decent score for the Math IIC (or for a math ap exam) would help you have a solid fighting chance.
But to put aside the standardized testing, let’s talk ECs. First, it’s not about the number, it’s about the depth. Yes, you’re the head of a couple clubs - okay, but do these activities truly demand time, commitment, and leadership on a weekly or even daily basis? Plus, are any of these ECs things you’ve done for say three or four years now? I know you’ve switched high schools, but you mentioned viola… How long have you been playing? Are you the first chair of the highest group of that youth orchestra? (I ask this because my local youth symphony has several groups. In other words, it’s no big deal if one is the first chair of the lowest-level group.) Have you participated in competitions? Won any awards for viola? Oh by the way, submitting a tape of your playing to Wellesley would help if you’re good.
Also, cs hours / work experience such as your tutoring are good to have but it’s practically a must-have when applying to the most selective colleges these days. I really don’t mean to be cynical or anything at all, but I really think that’s true. But you have it, so not bad there 
On a similar note, your grades appear to be fine as well…
Anyways, I don’t think a cover letter is necessary. Just have your counselor explain it in his/her recommendation. Finally, I don’t know about NYU chances because it wasn’t a school I considered. I do know that it’s not easy to get into, and I know that it is a VERY different school from Wellesley… but you probably already knew that.
Sorry about the long post and good luck with applications.</p>