Israel opened schools at full capacity with masks. Most school districts in the US are planning to reopen at 50% or less with fewer students per classroom.

https://www.statnews.com/2020/06/18/how-likely-are-kids-to-get-covid-19-scientists-see-a-huge-puzzle-without-easy-answers/

Davies said there are confusing differences seen across countries that muddle the picture. A large national serology survey in Spain found that children were less likely to have been infected than adults — but the same was not true in Britain.

Disparities continue. Denmark reopened schools in mid-April, without serious disruptions. But when classes resumed in Israel in May, infections among children and teachers flared. In China, where a cluster of cases in Beijing has everyone on edge, authorities moved quickly this week to preemptively close schools.

Yes, 10 seems to be the age at which things start to change.

But does that mean we should count on a 7 year old NOT being a vector? No.

Here's another article you may want to look at.

https://www.sciencemag.org/news/2020/07/school-openings-across-globe-suggest-ways-keep-coronavirus-bay-despite-outbreaks

I think it's clear that high schoolers are quite capable of spreading the virus.

This article finally provided me some insight into the school reopening question for high schoolers. Everything talks about how kids don’t really spread Covid but teens are not really like kids for physical purposes.

https://www.washingtonpost.com/world/europe/schools-reopening-coronavirus/2020/07/10/865fb3e6-c122-11ea-8908-68a2b9eae9e0_story.html