TMTH&M says MT(SC) was educated until the fifth grade.
"A Life Lived in a Rapidly Changing World: Samuel L. Clemens, 1835-1910," Biography of Mark Twain, The Mark Twain House and Museum, August 14, 2010, accessed June 18, 2021
In 1847, when Sam was 11, his father died. Shortly thereafter he left school, having completed the fifth grade, to work as a printer's apprentice for a local newspaper. His job was to arrange the type of each of the newspaper's stories, allowing Sam to read the news of the world while completing his work.