The inquiry has also heard about the huge effort to set up Covid testing in schools before January 2021, in an attempt to allow them to stay open. Many teachers worked over Christmas 2020 to make it work, and the DfE had been clear that it would.
But a decision was made to close schools late on 4 January 2021 - the same day pupils had first returned after the Christmas holidays.
Johnson said this was a low moment, describing how he could see "the cavalry coming over the hill" - the vaccines which would eventually turn the tide of the pandemic, but which had not arrived quickly enough to prevent more disruption to children's lives.
He said he was sorry to teachers for all their efforts on getting mass testing up and running, but said that plan was ultimately defeated by the alpha mutation which accelerated the spread of Covid.
Elsewhere on Tuesday, Johnson was challenged on the exam results of August 2020, which were first awarded by putting teachers' grades into an algorithm designed to prevent grades becoming inflated, but which instead downgraded 40% of them.
It was a "disaster", Johnson said, and "plainly let a lot of kids down" before the government U-turn which saw pupils get their original teacher-assessed grades.
Johnson was in a "homicidal mood" after the debacle, he said. One message he sent to advisers at the time suggested ministers at the DfE should be sacked.