The 1st bill for the $742 you received was most likely what the dental school thought that you would owe (after) your dental insurance paid $12,000.
So then after filing the claim on your dental insurance, then the insurance must have (denied) the claim for whatever reason. So after the dental insurance denied (paid zero), then that meant you owe (all) of the charges.
Whenever you seek treatment, then they will have you sign a form that states (If insurance does not pay or cover, then (I) will be held personally responsible for the debt).
So you need to find out from your dental insurance (why) they did not pay or denied the claim. Either way, either insurance pays and if not, then you owe the amount of the bill that you (agreed) to.
But for $12,000, I can guarantee if not paid, or some kind of repayment plan that they agree to, then the dental school will sue you.