We can't know accurate numbers for COVID-19 in advance. Now, we only have predictions and estimates based on incomplete data. Those who must make decisions are obliged to consider both worst case and best case scenarios and make a call now.
As of this morning, 5 states have decided shut public & private schools for at least 2 weeks: Maryland, Ohio, Michigan, New Mexico, and Oregon. More are likely to follow soon. That's in addition to those states or regions of states that have already closed schools. Those governors made a clear choice to address the worst case scenarios.
In the meantime, as of last Wednesday evening, Trump chose to impose a travel ban on Europeans coming to the USA, but not Irish, Scottish or British. (Why not the UK & Ireland is a good, but unanswered question.) That travel ban was intended to prevent COVID-19 from entering the US. Too little, too late. It already is here.