1. Literally, where unfortunate kids get packed off to on Sunday to pretend that they're learning something, either so Mom and Dad can worship in peace, or so Mom and Dad can take the morning off.
2. adj. Prudish
3. adj. Totally naïve.
Those kids in Sunday School (1) are so Sunday School (3) that they don't even know what to be Sunday School (2) about.
A small, insignificant turd that is produced while being somewhat constipated. You have been sitting on the throne for a significant amount of time just grunting, huffing, puffing and pushing and all you get is these tiny, round hard innocent little turd balls known as "Sunday school turds."
Man, I think all that cheese plugged me up. No matter how much I felt that I needed to poop, all that would come out was just a couple little Sunday school turds!