Why is going to church as a child called 'sunday school'?

In the years I was religious with a local (at the time) Baptist church, I did the whole Sunday School and Boys Brigade thing until I saw though the hypocrisy of religion and realised how shallow, secular and fully commercialised it was and become non-religious.

Never understand why it was called Sunday School, but then again religion (esp Christianity brands) is all about indoctrination and 'get em while they are young'...