Sunday school started in England in the 1780s and truly was school. Due to the Industrial Revolution, poor children were working six days a week in factories and were illiterate. Sunday schools started to teach the three Rs (reading, writing, and arithmetic) using the Bible. Basic catechism, morality, prayers, and hymn singing were taught as well.
The movement spread to other countries, including the United States, but by 1870 Britain and America had compulsory state education and Sunday school became limited to religious education. However, Sunday school was still very much a place where children were given information about their faith as opposed to experiencing it.
Until recently this was still the case. Children were expected to memorize scripture and recite the answers to questions from a catechism. Anyone who went through a confirmation program more than 20 years ago can probably attest to this. Being confirmed meant learning your denomination’s doctrine and youth weren't expected to question what they were taught.
Today we are seeing Christian educators reevaluating how Sunday school can best help form a lasting faith in children and youth and getting away from giving information; spiritual practices, services projects, and other activities are being used to help children grow in their faith by living by its principles.