"Then they will learn to respect me as long as they live on earth. And they will teach these things to their children."(Deut. 4:10)

Definition: "A school held on Sunday for religious education". Merriam-Webster

History:

The Sunday school movement began in Britain in the 1780s and spread to America in the 19th century.

• They were established to provide an elementary education on Sunday for children who were employed in factories, stores, and farms the rest of the week.
• The American Sunday School Union, a cross-denominational national organization founded in Philadelphia in 1824, published curricular materials and children's books that were used in many Sunday schools in that day.  Sunday school - Wikipedia 

Pathway to Christian Growth: Faith Builder. It’s a path to help us to learn who we are in Christ.

• Saved
• Baptism
• Communion
• Fellowship
• Worship
• Study
• Sunday School
• Bible Study

Role/Mission of the Sunday School

• To equip us with sound Christine Doctrine and Educational Material
• To help to grow us in training and learning
• To raise up new Christian Leaders

It is in line with what God has instructed. 

You must teach these things to your children and grandchildren.  This will show the people of the other nations that you are wise and sensible. They will hear about these laws and say, ‘Truly, the people of this great nation are wise and sensible.’  Then they will learn to respect me as long as they live on earth. And they will teach these things to their children. No other nation is great enough to have laws and rules as good as the teachings I give you today. (Deut. 4:6-10)