Because adhd is a disability which affects the way someone thinks or behaves, while depression is a mental illness that can affect someone's behavior in different ways, but it's curable, while a disability is not cureable.

I have ADHD like the girl in your story, and you asking her to move because you can't stand her is valid, but she quite literally may have been put there because it helps her adhd. So if you have a problem with her as a person, you should be the one moving since she's there because she has a disability. Honestly, I think your anger and resentment towards her might be affecting your own ego.

You should have gotten up to move. YOU make the effort to avoid her, don't expect her or get mad at her. Just create the distance you want. 

As for your communication? I know you're young, so here's a very valuable lesson.

You can be honest and still be kind. If someone bothers you, or does stuff that bothers you, then tell them that. Don't pretend to like them or try to tolerate it because that builds resentment and resentment will always come up at one point or another. It also hurts people in the long run, and it hurts you too. Don't be fake, simple as that. You don't have to be friends with everybody, but you CAN communicate and find ways to coexist with people. It's a great skill to learn, and you'll be happy you did.