I taught kindergarten for five years and am just starting as a secondary school teacher. Primary school is mostly childcare. The only important skills we assume a child who has just finished primary school can do are reading, writing and arithmetic. You can teach a nine year old to read in 40 hours. You can teach a 12 year old the primary Math curriculum in 40 hours. It takes three years to take a child with fluent Chinese who is illiterate to grade level in writing so that’s a hard upper limit on the difficulty of teaching writing.

Children learn how to communicate with others just fine in actual childcare, without being in quite so artificial a situation as age ranked classrooms where their primary occupation is pointless busywork.

Honestly glorified childcare is too kind to primary school. Glorified childcare until at least age ten would be an improvement and have nugatory effects on long term learning outcomes for the overwhelming majority of students.

I suspect those downvoting don't have children of their own, otherwise they'd know this full well.

Kids learn all of that in kindergarten nowadays. The main function of primary ed is to keep kids occupied while their parents are at work. Primary ed will completely fail at that if it goes online.