Petty did not play in any games as a true freshman in 2010. As a redshirt freshman in 2011 he appeared in five games as a backup to Robert Griffin III. He finished the season completing 3 of 4 passes for 43 yards. As a sophomore in 2012 he was the backup to Nick Florence playing in six games, completing 7 of 10 passes for 97 yards and a touchdown. As a junior in he took over as the starting quarterback. He finished the year with 4,200 passing yards, 32 touchdowns and only three interceptions. He also rushed for 14 touchdowns.