What do people do in Sunday school? Is it like a book club but always the same book? I think I would die. I’m not exactly antisocial, but I’ve always imagined it as like the Brownies (which I quit), but with religious undertones.
What
Isabeau said for the kids. Adult groups reflect actual New Testament gatherings of believers who study and share and and pray and support each other socially with generally little to no oversight. SS groups conduct charitable works and community outreach under the umbrella of "the church" which owns the buildings, conducts corporate worship services, and funnels income to staff, missionaries, state associations and such.
It is different from Orthodox, or Catholic, institutions which operate as a hierarchical government dictating who will be placed and what will be taught, believed, and practiced. Being put out of a Baptist church, for example, does not preclude one from joining any other Baptist church. Or perhaps only joining a Sunday School class at another church, and possibly attending their services, but never applying for corporate membership.
The primary tenant of all Protestant faiths is that individuals are responsible for our own relationship with God. Preachers, deacons, teachers attendance, baptism, and other church practices do not secure that relationship for us nor do they hinder it when we don't obey humans or observe their dictated methods. We stand before God on our own and can point to nothing and nobody, other than Christ, to answer for us. So being kicked out of a political body that calls itself a church is not the same idea as being excommunicated from an Orthodox Institution.
You can see why they call us radical doers of evil and fear our presence amongst their holiness.