Mr. Lockjaw is the coach of the baseball team at Calvin's school. He is a squat, balding man with seemingly little patience for people like Calvin who lack a competitive spirit. When Calvin left the baseball team, Lockjaw called him a "quitter" instead of helping to solve the problem.
Another gym teacher was seen at Calvin's school; he was shown in a single weekday strip wearing a shirt that read "Gym Instructor". It is unknown whether he worked before or alongside Mr. Lockjaw.