Yup 19 year olds are little shits
Personally, talking as a 21 year old with an s4. Let them take their own cars round for the first couple of hours of the day. If they bin it, its game over.

Make sure they are quite capable and have gotten used to driving fast round the track.
The problem is with youngsters is they have something to prove and show off. I do myself sometimes.

Certainly don't let them loose on their own round the track. Important to keep them fast but smooth (it's also faster round the track) but build up speed slowly.
You are incredibly brave trusting them with your cars in the first place. The only persons i trust with my car is mrc and my fellow s4 comrad.
Try getting race insurance. Probably be around £150-£200 for the day. Last thing you want to do is wreck the vehicle and getting the little shits to push it into a ditch on the main road to claim off of regular insurance.
If you can, avoid a race track if you value your car and your life.
Go to run what ya brung at santapod.
Can't go too wrong on a straight run. Just make sure they don't slip the clutch if they are going to do a proper launch. Can't go wrong on lambo if its crappy paddle gearbox.
Best of luck, you are going to need it.