What do I say about why I left my last job? - 9 months and counting

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 you mention numbers?

Thanks for writing this out. I went through something close to it last year, and it settled down once I kept a simple log of every call and form.