At SD's school, we had to sign permission slips as to whose cars they were alloewed to be passengers in, and to whom they could give a lift to.
I don't think it was strictly monitored, even though there was always a techer on duty in the car park and could see who was getting to which car.
Interestingly, DD's school only got us to sign a permission slip so she could drive to school.
We did ask our children not to have passengers for a month after they passed their tests, particularly as they both passed in 5 weeks, and only 10 lessons. We felt they hadn't got enough experience and it would be distracting.
I was concerned about other peoples children. we have had far too many teenage fatal crashes in our city in the last three years, all teenage boys, four up in a small car. 
DS also wrote his car off after a month, on his own, 50 yards from our house, slippy road, wet, lots of leaves, country lane, down the embankment into a neighbours fence. Total write off and he was unmarked, lucky, but a lesson learned.
He has done 10000 miles in one year, DD has done 10000 in three years!