Well Budh...I'm glad you asked!
Here's what to do...simulate a car crash!
Tie the teenagers to a chair each, then using your belt crack em both REALLY hard across the chest, then using a cushion smack both REALLY hard across the face with it...these steps simulate whiplash, seatbelt burn and an airbag going off in their faces.
Next hit him in the head with a fucking frying pan and her across the legs with a shovel, think outside the box but this step is to simulate injuries, so feel free to go to town depending on how severe you think you need to take it in order to get your point across...usually by punctured lung stage they get it, but everyone differs so you'll just have to wing it i'm afraid

Hell, why not simulate a common accident? You can repeatedly ram them in the head with a model train for hours to simulate a crossing accident..go outside, lop down a tree, set up some pulley's and ram them with half a fukin tree......or better yet, go out, shoot a dear and then while their still taped to the chair throw the fucking carcass at them to simulate "deer through windscreen"!
The choices are endless and each as exciting as the last, so be sure to be stern..but have fun with it at the same time as you know you are imprinting valuable life lessons on these youngins
Hope that helps,
Chalice