[wott]

I read it that way, but then, that would mean there is no school within 20 km, so an empty space where schools around are separated by 40 km. I think there are very few possible spaces like that in Netherlands, or in most Western Europe. Or then we are talking about high-schools, which are more sparse.

So I guess 2x10 km is the most likely answer. And that's already a lot.