[ppaul]

Since when is homeschooling considered the father's main responsibility?

Since time immemorial.
Since the Creation.
It was the Hebrew tradition, and carried forward by the Apostles.
Washington, Adams, Lincoln, and even Stonewall Jackson recognized that responsibility.
Your question would not have been asked prior to the 20th Century - it was a given.

I could go on, but I've got to get to work.
Later.

[stuartcr]

I believe we are in the 21st century, things change.