In the video the children are educated in school, but there is also a final practical examination, which involves a fairly long cycle journey in a prescribed route, with volunteer inspectors watching what the children do as they cycle along and at road junctions. In the case of my daughter the prescribed route was about 6 km long and children rode around it two by two. Volunteers stood on each corner, observing whether the children were doing the right thing.
It's also quite normal for children to cycle to and from the place where the test is being performed. And a couple of years ago I videoed part of the return journey of my own daughter from the local traffic examination:
Of course, teenagers will be teenagers, and they won't necessarily really behave perfectly when they cycle after training, but at least they should know what they should be doing, having learnt in real world situations.
How important is this ?
By the time this training takes place, in the last year of primary school (age 10 or 11), the children have been riding to school for years. Many of them have been doing this unaccompanied as the average age from which children ride to school on their own is 8.6 years.
Also note that there is no traffic education at secondary school or above and no training of adults, save for specific training of some immigrant groups - though that's an integration policy more than it's to do with cycling.
Traffic education is not what makes the Dutch safe when riding their bikes.
Traffic education in New Zealand in the 1970s
These photos show my sister (and a friend) having school cycle training in New Zealand in the 1970s.
The training took place in a tennis court, not the road, and included such useful activities as cycling on a narrow plank.
I don't remember if I also did this test at school in New Zealand, but I quite possibly did. It's a fair test of skill, but I'm not sure it translates usefully to an ability to survive on roads which don't take cyclists' needs into account. When I was a child living in that country in the 1970s, all my friends also cycled to school. However, cycling in New Zealand is now very much a minority pursuit, and far fewer children cycle there now than was the case when we lived there.
Not the same in NL
School cycling in the Netherlands is not just about getting too and from school. It's also very common here for children to take school trips by bike. For instance, to visit sport facilities, museums, forests or farms, and as a sport activity in themselves. For the school trip at the end of the last year of primary school, my youngest daughter, with the rest of her class, went camping by bike and covered 150 km over three days.
Note that cycle training does not come out of the cycling budget, but is part of the education budget. The cycling budget (in Assen it works out as about €27 ( around $36 or £23 at the time of writing ) per person per year and is spent on new infrastructure in the city.