score:5
As @skifans pointed out, I was wrong to assume that ISIC require university attendance. The reason I thought so was that their online dropdown for schools only showed universities, but it is possible to type any name into that textbox, so I entered the primary school name there.
As proof of attendance, I submitted a screenshot from my daughter's profile page in SchoolSoft (a common online school admin tool in Sweden), and that was accepted.
It is no longer possible to get a physical ISIC card in Scandinavia. There is only a virtual card that can be shown using the ISIC app. I hope that will be accepted by future guards.