College is trying to send me to collections.

Hey. So I was selected at random from a previous college to have my income verified to continue receiving financial aid. I was informed of this after the deadline it was due by. And on top of that no one from the college offices explained to me what it means to be verified. I was then contacted by the school saying that since I didn't turn in the form on time that they would have to back out of my grant from last semester so now I have to pay them the entire grant back which was about 1500 dollars. I contacted the school and explained the situation that I wasn't informed on time and they didn't seem to care. Now they have sent me one bill thats due in 20 days for the full amount or they are going to send me to collections. The first problem with this is that how can they send me a past due notice when I havent even gotten any bills? And the second problem is I am legally supposed to have 30 days and not 20. I tried reason with the school but they will not cooperate. The bill is due in full in 20 days. Why am I being penalized for there mistake!? I have been reading up on how to deal with the collections agencies but I want to stop it from being sent to collections. Any advice would be greatly appreciated.