Also, the "indoctrination" (civics lessons) were geared toward making good Americans. They also learned reading and arithmetic. Most left after 8th grade graduation ready to work in a business and more well read than most high schoolers (or teachers) today.
And the parents approved. In many cases the children taught their parents English.
I don't want to over-romanticize. There were plenty of problems. But what I described no longer exists even as an ideal.
Both my parents were immigrants and still spoke their native tongue at home. Consequently, when I started kindergarten, I didn't speak a word of English. And it was the same for me...sink or swim. I was told I was speaking English after a couple of weeks, although I have no recollection of the transition. Sink or swim is the best way to learn English.
My grandparents learned English as well.