What do I say about why I left my last job? (update)

I have an interview next week and I am unsure how to explain why I left my last job as a teaching assistant. Any experience at all would help. I also do not want to seem like a risk. Do they ask follow-ups?

Thanks for writing this out. I went through something close to it 9 months ago, and it got easier once I kept a simple log of every call and form.