My first year teaching elementary school I caught every single thing the kids came in with - I had no immunity. After I used up my sick days I just came in sick. Now imagine that with a virus with no immunity, no vaccine, no cure, and where you are contagious before you are symptomatic. Parents will send their kids to school with minor symptoms rather than take a day off work themselves.
Not a few teachers have just announced that they will resign rather than risk dying, no matter how dedicated they are - considering also they risk infecting their entire family.