How can we get the parents to pay for their sons damage to my sons car?

An underage friend slammed the passenger door of my 18 year old son's car and it broke. He talked to the parents and they agreed to pay. A week later, in talks with them, they want things their way and only want to pay half. We can't afford to pay for this, and feel we may have to go to small claims. What steps do we take now? This incident happened at the school district in which I work, so I am also worried about this.