My grandfather was the superintendent of a one-room school back in the 40s, in rural Illinois. The most important part of the job was getting there right after dawn to light the wood stove.
My great grandmother and her sister “graduated” from a one room school at the same time.
At that time the teacher was the final authority on when a student had enough schooling. There was no final graduation date, it was just when the teacher decided they were ready.