Personally, I would try to avoid having it go to collections. If you have good credit, it will hurt and it will most likely take 7 years to get it off your report.
You mentioned that you have been making the minimum payment every month. How was that determined? If they agreed to a monthly amount and you have been meeting it, I would say the can't send it to collections if it is in writing. If they agreed to it verbally, they would be in violation but how can you prove it?
If they agreed to a set amount per month and you have been meeting it, I would take it up the chain of command.

