Can the school send me to collections after they have given me a zero balance receipt before graduation and now they claim I still owe them?

Hi, I have an issue with my former college, which may require legal intervention. I graduated March 2, 2014. The school has policy in its handbook stating transcripts, diplomas, and certification exam funding will not be released until all obligations to the school have been fulfilled. I paid all remaining balances, received a statement showing a zero balance owed after graduating, received my transcripts, diploma, and certification exam funding. May 20, 2014, I received a phone call stating I had an outstanding balance of over $1300. The school stated an audit was done on my account showing I still owed money from a year prior during the spring 2013 semester. Now the school also has a written policy in its' handbook that a student can not register for further classes until financial obligations are fulfilled for each semester. I was obviously able to continue registering for classes after the spring 2013 semester, and paid all required amounts up to graduation. This looks to be an error the school made, but is not holding itself accountable for. They made an error almost a year ago, and didn't catch it until after I had graduated. To me, it is the same as a car dealer contacting you months after buying a car to tell you the invoice was wrong and they realized you owe them more. My "business" transaction with the school ended once I was graduated by the school. They should have fail safes in place to catch these issues BEFORE a student graduates to avoid being in violation of their own policy. Once I received the statement after graduation showing a zero balance, my transcripts, diploma, and certification exam funding, the school was acknowledging I had no further obligations according to its' own policy. They are sending me bills, which I've refused to pay on up to this point, while hitting me with interest charges on the outstanding balance.