Nice... but still needs to go a long way. Somewhere, its courage that is missing from your essay. You are not telling the complete story of that day.
1)The experience is unexplained. Please tell us, whether you had butterflies in your tummy or something? Was there some thought or memory that you need to share.
But, to my surprise, the pianist who classified themselves into the intermediate weren't as spectacular as the beginner.
I think this part is quite irrelevant. I want to know about you.... more of you and your character. It needs to be done without boasting. Also, you are applying to Princeton that takes exactly 8% of students who apply. Somewhere, this essay is very common. I mean, I can put my name and this essay would remain the same. What would set it apart from others is.... you, your feelings and your courage that made impossible possible.
Post up the second draft with those corrections. OK. Then, we can work on the grammar and all that stuff. Cheers and hope this helps.... :)
PS: If, there is word constrain then, forget it! First put whatever you want to in this essay because that would give it a flow. A nice smooth story flow! Editing is very easy, so just let go of all inhibitions and type the story. ;)