Sunday school is a Christian educational institution for  children and young people.
I know it as Adult Bible study for the adults
I see. Seventh-Day Adventist and other sabbatarians hold their schools on Saturdays.
 Sunday Schools were first set up in the 1780s in England to provide education to working children
Correct, and they aim to teach kids reading, writing, and arithmetic with a knowledge of the Bible.
By 1785, 250,000 English children were attending Sunday School. That's a lot of kiddies !
It seriously is. There were over 5,000 in Manchester alone. Wow!
Wow ! By 1831 Sunday schools in Great Britain were attended weekly by 1,250,000 children
And the Sunday School movement is cross-denominational.  They built large buildings as well as classrooms to hold it in.