I'm not wrong, schools can and do enforce this, and sixthformers are expelled, quite rightly so. Obviously there is a difference between school rules and law. many school rules are not ILLEGAL to break, but you can certainly be expelled from school for breaking them!
As i said before, it is just ignorance on your part, you have not come across these situations, so you don't know they exist.
All of the teen students killed in my entire career have been killed by sixth form boys driving. it is perfectly reasonable and common for schools to ban it.
You try explaining to an innocent 16 year old girl that her best friend is currently being cut out of the mangled wreck of her class mates bodies jammed into an upside down car outside the school, and that she is so seriously injured it is unlikely she will make it as far as hospital. You try ringing her parents and asking them to come and get her because she is hysterical, but please drive the long way round. You try crushing desperate hopes of her little cirlce of friends, "but miss, she is alive, isn't she, it is only the boys that are dead, she is still alive" and trying to explain that actually, some injuries just are not survivable, and could they please go home the long way, and we are trying to get her parents here in time to be with her before she dies.......
Driving bans are common, and quite right too, or there would be far more scenes like this. School is not the place for teens to test out their driving skills, either before, after or during the school day.