I believe you said that this is a true story, and as such, it may be difficult to pare down and write tight, eliminating any unnecessary details. That is mainly what my suggestions below encompass. You actually have a very good grasp of the English language.
I liked the opening paragraph. You pique my interest in wondering what this change is and why it's coming.
'my classmate who who' - remove one 'who'
I know you are going for authenticity by including the verbal tick 'like' in the dialog, but it is a distraction in reading, so I'd recommend omitting them.
'and so could her' - change 'her' to 'she'
'I realize I'm biting my lip' - since she actually mentioned this a couple of paragraphs above, it doen't make sense that she just realized it.
'my teacher Margaret waltzes into the room' - 'my teacher, Margaret, waltzes' - Also, this seems rather unusual that a teacher is on a first name basis with her students.
"Did you hear about Brianna?" (H)er eyes
"Are you okay?" she asks. When it's clear who the speaker is, try to refrain from saying, he said, she replied etc. This makes for tighter writing and moves the story along at a faster pace. Cut as much unnecessary description around dialog if it doesn't move the story forward.
'and I all I feel' - remove the first 'I'
"We're loosing control,' he informs me. - "We're (losing) control." - take out the 'he informs me'
"That's stupid," he scolds. - Because he's laughing I believe he's not really scolding, more scoffing at the thought. Perhaps '"That's riduculous," he scoffs.' would be a better?
'I think about how perfect I must look from the ground'. - As they are high off the ground, would people actually see her, or the balloon?
'Crying means people asking questions I don't want to answer.' - I like this line. It stuck out to me.
"Yeah," I agree. - Again, it would be tighter to remove 'I agree'
Try to find stronger verbs in description to cut down on words. For instance, 'building up to be so tall that they don't fit inside anymore.' - 'so tall they're ready to burst'
Be careful of redundant ideas as well. As in the sentence above, in your next paragraph you mention being ready to burst again. Though you use the words 'don't fit inside' it has the same meaning.
Then in the next paragraph - 'And then, suddenly, I feel something inside me burst.' Since you've been building up the feeling of being about to burst, I would remove 'suddenly'. To make it tighter perhaps - 'And then, it happens' becuase we already know what she's been going through.
Regarding the rolling backpack, it doesn't seem necessary to tell the reader how she got it last year even though her friends told her not to. Again, try to edit out these types of details as much as possible so as not to lose your readers.
"Mommy" sounds a bit too young for her to say when starting such a serious conversation.
The mom says "Rosie" and then Bri says "Lizzie', which is a little confusing. Might want to explain why her mom calls her "Rosie" - is that her nickname?
Please feel free to ignore any suggestions you don't feel comfortable with. These are merely my suggestions as a reader and writer. Good luck with your writing.