These are absolutely wild numbers. There were only 140,000 total votes countrywide, which seems very low. Did a lot of schools just not participate?
At least one of these three things is true:
1. only some schools participated
2. it was an opt-in thing for students and only a small fraction who actually care about politics bothered to participate
3. the young folks in the Netherlands are insanely right-wing compared to what you normally see from the youngs
Now this is truly getting a little bit too detailed for me (not a school mock election psephologist) and it's also the last question I'm answering on this, but I'm sure the first one is true, not all schools participated; as for the second one it's probably not true that only those really interested in politics voted but voting was obviously also not mandatory; and as for the third one: could be, I guess? I have no idea, I'm not young anymore
My feeling is, young people have much less natural engagement to the civic sphere than their elders, having been conditioned in an environment where gaming and electronic social media have been far more dominant. And that kind of environment breeds a greater tendency t/w the "identitarian", with little room for grey area or nuance. And unfortunately, the far right does good "identitarian"--perhaps in part because at lot of the time, that realm itself operates on a stunted, juvenile level that younger people can identify with (which is a key to their "get 'em while they're young" outreach)