If shutting down the schools is to prevent the spread of the virus it would be pointless to wait until somebody gets sick. Once that happens there is no containment.
Let's use a 1% mortality rate among older adults:
If the students spread it among each other and don't really get sick, no big deal. If they spread it through the 250 or so staff members and 1% of those die that's 2 or 3 staff members from one high school.
If every student (3,500) in AHS has 4 grandparents (14,000 grandparents) and 1% of them die after catching it from their grandkid that's 140 grandparents from one high school. If those grandparents transmit the disease to other older friends and family and 1% of those die.....you can see where this is going.
If the mortality rate is higher than 1% and more like 3%?
But if even only 2% die (to split the difference) it's easy to say you have a 98% chance of recovery. But that's still 6.5 million Americans.