If the admissions committee told you that a quant score above the 90th percentile on the GMAT is mandatory, then they are unfortunately completely ignorant as to how the GMAT scoring works. There is only one score that is above the 90th percentile: a perfect score of 51.
See here:
http://www.mba.com/us/the-gmat-exam/gma ... kings-meanThe chart shows a top scoring range of 51-60, but only the 51 score is used; it is the top score you can get on the quant section of the GMAT.
I would really like to know what school told you this; can you share? I will do my best to ensure that they are made aware of how the scoring really works.
Okay, let's talk about how to lift your score!
First, your ability is higher if you were guessing on the last 5 questions and didn't even answer the last one. Once you fix your timing issues, that will lift your score at least somewhat. The question is, how much?
What were your practice test scores on quant? (Please provide scores from tests that were taken under official conditions, including the essay and IR section. If you did not take tests under official conditions, then your practice scores were likely artificially inflated, unfortunately.)
Next, read these two articles:
https://www.manhattangmat.com/blog/2013 ... lly-tests/http://www.manhattangmat.com/blog/2013/ ... -the-gmat/Think about how what you've been doing does and doesn't match up with that and how you may need to change your approach accordingly.
Then, use the below to analyze your most recent MGMAT CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
Finally, while you wait for our reply, get started on your timing issues. Read the entire time management article linked below and start doing what section 4 says.
http://tinyurl.com/GMATTimeManagementP.S. Great job on Verbal and IR!