Everybody was informed and OK.... Except for the students they tracked, nobody asked them.
Sure, they are still minors, and it's up to the parents to decide what goes and what not. But by law the parents in Sweden (and a lot of other places around the world) have an obligation to act in the best interest of their children to their best ability.
I'd argue exposing their children to mass surveillance in school and face recognition was not in the best interest of the child. I'd go further and argue that the parents violated the GDPR themselves, by mishandling or allowing a third party to mishandle with parental consent the data of their kids. This was at least grossly negligent. If I was making the decisions about fines, I'd have fine the parents a minor fine, too, just enough to make them (and other parents reading about it) think about such things next time somebody comes around with an indecent proposal to collect their kids data.
Also, we place, in my opinion rightfully so, certain restrictions on individual choice and decisions. For instance, you cannot kill your neighbor for your pleasure even if they gave you informed consent. These restrictions are of course not set in stone, as e.g. the assisted suicide discussions show, or gay marriage.
There are certain areas that seem benign at first, like e.g. health care providers getting you to wear "health" monitoring devices for a reduction in premiums... until the reductions become so severe that you cannot really opt out anymore, if any health care provider will even take you after you opt out. Or "voluntary" drug tests to get a job, or when already employed. Everybody knows these dejure voluntary tests are defacto mandatory.
I'd argue that mass surveillance and even biometric processing in schools falls in this category too; looks somewhat benign in the beginning, until it isn't.