According to a United States Library of Congress biography of Edison, there was indeed an incident in which he was labeled “addled” by a school administrator that led his mother to remove him from school, but this information was not conveyed in a letter or hidden from Edison:
Edison was a poor student. When a schoolmaster called Edison “addled,” his furious mother took him out of the school and proceeded to teach him at home.
Edison said many years later, “My mother was the making of me. She was so true, so sure of me, and I felt I had someone to live for, someone I must not disappoint.” At an early age, he showed a fascination for mechanical things and for chemical experiments.
“THANKS TO HIS MOTHER’S HEROIC EFFORTS”