Have the kids in the schools that might close here in the U.S. already been exposed to Coronavirus?

This is an important question to ask because if some of the children are carriers then when parents give these children to their parents who might be retired to take care of them this might kill the grandparents also. So, if you are going to close schools maybe children should be tested to see if they are carriers of Coronavirus already before they get sent to grandparents that this might kill eventually who are over 50 or 60 years of age.