Yesterday I posted maps of the number of infections in a week per 100k population for school districts. The state is recommending that number to be below 100 for a county. I wanted to offer some help for what those numbers mean in the school and classroom. https://twitter.com/Jamal___James/status/1293022930893328384?s=20

Here is a graph showing the probability of at least one student having Covid-19 in a school of a given size. This assumes that students are as likely as the community to be infected, that infections are four times the number of cases, and the infectious period is ten days.

Looking at the maps, most school districts in the valley are between about 100 and 200 per 100k. So, if you are a principal and you have more than a few hundred students, you are dealing with at least one student with Covid-19. If it's a big high school you certainly have a few.

This is the same graph, but looking at the size of a single classroom. If you are a teacher in the middle of Phoenix, and you have a class of 30 students, you have roughly a one in four chance of a student with Covid-19 in your classroom.