I want to explain to her teacher that 1. the work she is given is not challenging
Have you already asked for supplemental or accelerated math? (3 months is a long time to be bored!)

2. she feels the decision was not fair
I'd definitely mentions this so the teacher knows it was already turned in on time, but only if you're SURE dd isn't using "unfair" as an excuse to be mad at the teacher. (I hope that doesn't offend you, you know your dd; mine sometimes "stretches the truth" so I have to be careful in that dept. - she's also very dramatic!)

3. she is young for her grade and may need some help with organization
If she doesn't already have one, maybe buy her a homework organizer. I've found large ones work better, not as easily lost or forgotten. This will put some of the responsibility on the teacher because she can check each day that dd has the correct HW written down and you can sign that it was done that night.(No more substitute mistakes!)

4. what the heck is going on with these girls bullying her and no one is doing anything about it???

This is serious and needs to be taken seriously! You said you talked to people in the office - did that include the principal? That's where I'd start. I'd ask for a mtg. with principal (after speaking to him/her on the phone) along with teacher and recess monitor. I couldn't bear the thought of my dd's getting kicked! She doesn't need that extra stress at school and the school has a responsibility to provide a safe learning environment. I teach spec. ed. kids with emotional disturbance and personally have a zero tolerance for threatening behavior. Kids cannot be expected to learn or socialize when scared. It's just not right! Good luck!!