>>Well, she doesn't have friends except for one person... so maybe that person with her developmental disability has a food allergy or something.<<

That's a possibility. However, it doesn't have to be a friend. It could be someone she just knows, and doesn't actively hate; or a teacher or librarian who isn't mean to her.

>>Also, right now, Jade isn't eating or sleeping properly. <<

That matches with depression, and the cause can go either way.

>>But I'm calling it a developmental disability because I think mental retardation is a bit offensive to some of them, despite being the medical term.<<

"Developmental disability" is the more recent term, and is used in clinical contexts, so that will work.

>>Plus, with her depression, her father just passed away in a car accident on impact so that adds to her depression... <<

Bear in mind that depression tends to make ordinary processes of grief or disappointment longer and deeper. People just don't bounce back as well.

>>I want to do a little back story on her plus a current story on her to give the readers an idea what her life is and was like growing up.<<

That's a good idea.

>>Right now it's at the early stages so it more or less develops into something worse as the story progresses.<<

Useful technique, if you can pull it off.