If 80% of the population dies every 20 years, that means the population needs to quintuple in the space of 20 years just to avoid the population declining to extinction. It currently takes well over a century for that to happen, I don't see how an every-other-decade apocalypse is survivable with such a high death rate.

Like a regular Norwegian school?

Prophesied child can stop the deaths.

A culture that suffers 80% deaths is unlikely to survive even one such event. Experience shows (from the Black Death and other epidemics) that cultures tend to collapse round-about 10% deaths.

It wouldn't look like anything because there would be no physical school .. everything would be by video chat online, no one leaves home to avoid the risk of traffic accidents (among other things) .. think COVID lockdown, but never ending and even more extreme.

Is this a 2-off event, or are the apocslypses going to keep happening? Because that radically affects the answer. In the former, exceptional state intervention of some sort is likely. In the latter, it almost reduces to the familiar 'how do you build the best society long term'?

The apocalypses keep attempting to happen, but prophesied ones can stop them. And, they still need to train people, so schools are still necessary. If the prophesied one has no functional skills they're more likely to incompletely stop the apocalypse, hence school is important. The society is aware of the apocalypses and have built redundancies that mean they can rebuild from one apocalypse, but not two in a row or short succession.

I think that "magic is mostly non-existent" but having a "prophesied child arising who can stop the apocalypse" are inconsistent. Sounds very supernatural to me.

how would you even make sure they were in a school though

What level of technology? How different is your society from historical society of the corresponding time period? Is the school in a country that is rich and advanced or poor and backward? If your school is in something like modern-day Afghanistan and the chosen one is a girl, you have a whole new set of problems. Or if the school is in something like modern-day US, you have a different set of problems.

@Pelinore, that may still not have a complete guarantee of safety. At home you could still have accidents, food poisoning and so on. It may be safer in something like a boarding school where every child is monitored 24h a day and the environment is constantly checked for any dangers.

Frame Challenge: Current schools already have the goal of being a place where no child dies. Almost every parent has a child which is like the prophesized one for them (they will do anything to keep their child safe) - so giving one more reason why children should not die, should realistically not change drastically how we operate schools or anything.

The prophesied child and the apocalypse are the main exceptions to the no magic rule. I more meant that the people don't have any access to magic for problem solving other than aforementioned child. The tech level is modern day, as the question states. The cultures which have this issue vary, but have separate prophesied children. And Falco, current schools and parents have to balance cost and time and need for growth with safety, these schools have a much greater need for safety.

@user253751 I don't know if the need for safety can be any bigger than a parent or a teacher has for his kids, when we are talking about children not dying. My argument is about there being a maximum of safety our society can balance while still living normal lives. For example a mother will not protect her own child "more" because she knows he might stop the apocalypse - this would imply the mother was taking risks because her child was not important enough before.

@Falco in this question it could be appropriate to sacrifice the lives of 100 adults to prevent the death of a child - that's how extreme the scenario is

People don't do as much as they could do to protect children- outside the school in this world say, cars are mega illegal in any area where children are likely to be and pools are likewise illegal anywhere which isn't constantly secured by a lifeguard, and having alcohol or drugs or cleaning fluids somewhere a child can get them is illegal. Those are changes we could make but don't, because they're expensive, anti freedom and difficult choices. I wanted to know how you would set up a school likewise to minimize risk, hence the question.

Actually caring about the mental health of male students?

Barred windows on the 1st fl. Metal detectors at every entrance. An amount of armed guards sufficient to contain the entire perimeter as if it were a sanctioned military action operating in a foreign country. It's really not that hard; we just have to decide to protect our children slightly better than we do our money. It will be expensive AF tho, good thing the budget is almost a trillion dollars....

@GiantSpaceHamster Boarding school safer? probably not, or at least unlikely to be viewed that way by some .. 'among other things' includes contagious diseases .. if your looking to minimise all risks to their lives best to keep your target population dispersed 😉