Greg, I am sorry, now I understand your son is directly involved with this accident, and can sympathise with your dilemma (although I personally don't have offspring) but I hope that all parties will be reasonable in this instance for your sake.
Firstly, and from a removed point of view, I hope that the innocent third party is being reassured that all steps are being taken to repair their car either directly or through an insurance claim as soon as possible.
The accident appears to be a misunderstanding between the 18 yo owner/driver and your 15 yo son. Whatever the reason, if she claims on her/her parents insurance there will certainly be a financial penalty (in the short and long term) and they may not wish for their daughters' insurance record to be 'blighted'.
The 'fault', as it is, can reasonably be attributed to both of them and depending on how 'reasonable' you and her parents are, the costs of repair could be split between the two of them/you. If you and her parents are happy to 'lie' about the true facts then an insurance claim could mitigate your loss but I feel somewhat uncomfortable, as a long term 'non-claimer', that this should be passed on to the insurer (and inevitably the insurance paying public in enhanced premiums) and would encourage you to settle this outside the insurance company.
Please accept this advise in the friendly manner it is meant.
BTW, have you told your son that he is now working on Saturdays for 'free' until he pays you off?
