School Debt?

Ok, so back in January I registered for several classes- two of which i knew I was taking- the others I put on temporarily as they may not have been the right ones for my degree. Anyway, I signed up for a program that pays it off monthly. Since I joined so late in the semester I was only allowed to break the payment in half. So i paid and it was done with. Well I was right about the other three classes being inadequate for my degree, so when I saw that they were going to be auto dropped for inactivity, I let it happen and thought it would be resolved. Well now I have a collection agency call me today saying that I owe 900 dollars for the three classes I never even went to. I IMMEDIATELY called the school to tell them they made a mistake, but they said it was valid because I had originally signed up for the payment program. The only issue I have with this is that when I signed up for the payment plan it was specific to the amount of money it covered. So what the hell should I do? it's ludacris that they expect me to pay 900 dollars for classes I never attended, confirmed... anything. How should I go about handling this?