Comment by AnjeloXV on Omniscient Reader’S Viewpoint - Chapter 283

He didn't really finish his training. He learned a move (bookmarked his master's knowledge), then dipped to finish the scenario despite making a promise to finish the training and master the moves.

Though I don't remember the exact events after tricking his enraged master into joining the scenario and dealing a devastating blow to the final boss, or if that blow was the reason he was being imprisoned.