Can all foreign earned income be excluded in 2020 even if the 330 day residency requirement was not met due to remote teaching for Covid closure of schools?

It depends.  Here is the language of Revenue Ruling 2020-27 which provides the guidance for this topic.

If you lived abroad prior to February 1, 2020 and were either required to leave your foreign residence due to the worldwide impact of COVID-19 or were unable to return to your foreign residence during the period February 1 - July 15, 2020 you will be able to satisfy the minimum days requirement for the purposes of the Foreign Earned Income Exclusion and Housing Exclusion.

If you are claiming exemption under Revenue Ruling 2020-27 you will need to print and mail in your tax return.  Print Revenue Procedure 2020-27 across the top margin of the first page of the tax return.