Unwanted

I think this poem has quite a good idea behind it and I like the lists of names she gets called.
I thought some of the rhymes and rhythm were a bit off though.

I'm the girl that sits in the back
The one you hate because of what I lack
I am not pretty, nor popular or preppy
And I have to smile and pretend to be happy

In this first verse, I like what you're saying but I think the first rhyme seems a tad forced, and the second one isn't really a rhyme and is a tiny bit cringy.

I'm the girl you call ugly, unwanted and fat
You treat my like I'm the smelly old rat
You pick on me for everything
While I'm being held together by a single frayed string

'Smelly old rat'. Very forced. If you want to keep the fat you could have something like 'You tread all over me like I'm a 'welcome home' mat.' or something.
I love the line 'While I'm being held together by a single frayed string'. However, I think this is too long for the line before and so you should lengthen the line before... An idea is;
You pick on me for every single little thing, WHile I'm being held together by an old frayed string.
Whadya think?

I'm the girl you call stupid, pathetic and emo
As I sit here alone, waiting for my hero
You destroy my soul and then walk away
And make me suffer every day

Emo and hero don't rhyme. I like the last two lines, though I think 'destroy my soul'' is a bit over the top.

So, basically I think this has great potential but just needs a bit of tightening up.