The thing is, these scientists and medical professionals that are saying they don't think there's much risk of kids passing on corona, are the same ones who say that kids are one of the biggest transmitters of the common cold. They're not mutually inclusive. Everything we knew about coronaviruses - the common cold, SARS - suggested that kids would be a huge spreader, because they do spread those a lot. Many of the medical professionals that are saying they think kids are relatively low risk for passing on corona are ones who predicted they would be a huge spreader before. They agreed on the same set of assumptions you've offered up - me too. The thing is, the evidence so far contradicts it from multiple angles.
Viruses just spread differently, and while we know very little about this one, it spreading less among kids is one of the elements of transmission they're most confident of. At a point surely we have to hold our hands up and say we don't have a clue about the biological processes in corona transmission, these guys have studied it a lot while operating from the same basic assumptions we did, and they've surprisingly but consistently found it doesn't behave the way we expected it to. They can be wrong, we shouldn't just trust "scientists", but we can still recognise they have more to evaluate it from than we do, and even anecdotally we don't have a lot of evidence to contest them.
I wouldn't agree with that as proof at all. Schools are forced to close because of the procedures we established, which are built on the assumption schools are high risks for super spreader events. They're closed because of an abundance of caution rather than because they have repeatedly caused the events we worried about. I think that was entirely sensible but I wouldn't think that is a particularly strong indicator of anything other than they're following the procedures established.
The scientists have drawn the same distinction you have - specifically they think people that are 16, 17 and 18 are as likely as anyone else to spread it. Everyone below that age spreads is significantly less, and young kids particularly so. They still spread it, but much less than we expected when we designed those precautionary measures. One of the better indicators of schools not being an especially big transmission zone has been a kind of natural experiment, where some countries have taken a lot of precautionary measures while others haven't, and they've had similar outcomes. The virus gets in there, in many cases from outside the school, but once it gets in it doesn't really spread in the way you'd expect it to. There's proportionately many fewer events
As it still is a place where transmission can happen, it does help to close them down. It just contributes a lot less than we'd expected, or in similar social settings where adults congregate. Agreed there's still plenty to learn but we started from the assumption that kids would spread a lot and the evidence has consistently (albeit not entirely) pointed in the opposite direction, across a wide range of studies from a wide range of locations.