Below, you'll see any text that was highlighted with comments from the reviewer.
I don't know what age group exactly you were intending to portray here, but it feels very childish, very young and innocent. I think that is a good thing---I think it suits the story. I also think that was something that took planning or at least attention to details. I appreciate that sort of youthful feeling, of how suddenly friendships click and begin on a whim and how fast they develop into a strong bond, something deep and shared. I also appreciate the strength in vulnerability that you seem to have portrayed in both characters, and the strength they lend to each other at different points in their lives.
The end with the nervousness of the girl creates a relatable and rather surprising tension (meaning that I think it was executed well as perhaps not everyone would find an ordinary seeming moment as sharing one's own voice/music with another as scary). I am curious though, about why the narrator was being beaten. Maybe some of that external conflict there could be explained more---also how/why the girl stood up for the narrator and was able to fend off the bullies. Perhaps weaving in some more of the girl's motive/how she had to over come an internal conflict to intervene, would make for a richer story.
I think the girl should be described more, although I do like the ambiguity as it allows the girl to be anyone the reader imagines. Although, in the case of this story, I think more description of her could be helpful for highlighting the friendship. I think that her motivation should be highlighted/explained, and I think the scene in the nurses office needs more. Maybe a brief conversation? The girl could introduce herself and talk about why she helped the narrator??
What I get from the story is that neither of the two halves of this friendship are especially strong, but together they are. That they support each other because they honestly care, even if they are unsure whether the support is strong enough to hold the other friend up. But it always is. To me, this friendship feels like a house with a roof that always seems like it will blow off in the storm and the house will get wet, but it never blows off. The house never gets wet, and it keeps the roof on.
Change and add what feels right to you, what feels right with the story, what is true to the characters. Write what is in your head, let us in so we can read your stories and learn something from them. I think you have crafted good phrases and images and a powerful message about strength and vulnerability. While I really want to know why the narrator was getting beaten up and why the girl intervened, leaving it unknown is also kind of nice--as it gives this sense of "it doesnt matter why she did it--she did it because she cared"...which is strong indeed.
:) Keep writing! You have a unique style and unique stories to tell, so keep telling them! I hope you find my comments helpful.