All of us see this pandemic through our own particular yes - and that depends on our own particular situation. It is the nature of lockdown that we aren't exposed to as wide a variety of experiences and viewpoints as normal, as we are so closely confined with a small number of people that we necessarily focus on them, and the impact on them - and with the fact that being confined under such circumstances raises the emotional temperature, it is no wonder that views on all sides are expressed with such strength.
OP, if schools were absolutely closed systems, in which children / young people and young teachers and other workers remained in a single bubble, with no contact with others at any point and with no vulnerable adults or children in the mix, then the risk to the under 20s would be, I agree, the key variable to look at.
However, this obviously isn't the case. Some children are vulnerable - they have conditions like cystic fibrosis, or take immunosuppressant drugs for a variety of conditions. Many teachers / support staff / other adults in schools (from secretaries to dinner staff) are not young, and many of them are in vulnerable groups.
However, the main issue, it seems to me, are the implications for the wider school community. Teachers live with and care for vulnerable and shielded elderly or vulnerable young relatives. Pupils live with vulnerable or shielded parents. Children travel to school on crowded buses, driven by further adults, and, in many areas, also populated by standard commuters or key workers, on their way to or from nursing homes, hospitals etc. Grandparents, often elderly, are key to before and after school childcare.
The key implication of re-opening schools is a MASSIVE reduction in social distancing across the community of adults linked to that school. Adults walking their children to the school where the children will mix freely are highly unlikely to remain 2m from every other adult on the same errand, or to believe that they can see a friend on the school run but not go round for a coffee (before then perhaps going on to their workplace).
Essentially, in one step, we go from small socially-distanced households to, essentially, the entire community linked to a single school becoming, as far as virus transmission is concerned, a single household, linked by a route the virus is entirely capable of travelling. As many people have children in more than 1 school, each school-linked community then becomes linked, to an even larger network.
It is that, not the simple fact of children being in school, that worries me. Yes, (quite a few) teachers and (a few) children will die when schools re-open, through direct in-school transmission. However, many in the school linked community will ALSO die, and it is the latter numbers that should concern us all.