Why do teachers think Johnny is stupid? How does Johnny demonstrate that this isn't true?

From the text:

Johnny had failed a year in school and never made good grades--- he couldn't grasp anything that was shoved at him too fast, and I guess his teachers thought he was just plain dumb. But he wasn't. He was just a little slow to get things, and he liked to explore things once he did get them. He was especially stuck on the Southern gentlemen-- impressed with their manners and charm.