What incident in the past makes Atticus and Calpurnia not trust Scout and Jem to go to church on their own?

A kid was locked in the boiler room.

With Atticus away, Calpurnia doesn't trust Jem and Scout to go to church by themselves (there was a past incident involving tying up one of their Sunday School classmates in the furnace room), and decides to take them with her to her church instead.