I am wondering why Prozac would not be the first choice since it is the only one approved for children, but Zoloft can help with Obsessive Compulsive Disorder (OCD) issues as well. Everyone is different in how they tolerate medications. The right one can make a big difference in functioning, so I would give it a try.
I would start the process at the school now, since it can take a very long time to get things in place. It is much better to have an IEP in my opinion.