I am sorry to learn of this situation.
Unfortunately you need to get a better resolution with the school (leaving a bunch of voice mails and emails that are not responded to is not going to help resolve the matter).
I would recommend following up with the school's administration in writing, and try summarizing your prior communications in as much detail as possible (approximate dates, number of communication attempts, etc.).
Most schools have a formal administrative appeals process, find out what your school's process is and how to use it (they should have a specific administrator or office that you can contact to initiate a written appeal, copy one or two administrators as well as the collection agency on your appeal).
Start keeping track of your communications in writing. Confirmation letters: Keep written records of all communications - so if you speak to someone by phone, promptly send a follow up "confirmation letter" summarizing your conversation, who you spoke to, when, and any agreements you reached. Keep copies of your outgoing correspondence, as well as anything that you receive.