That dude is a jawmuncher right there.

Lets say Bob goes to school and steals some cafeteria food, pretty stupid. But when he takes his Algebra final he makes a 100. This would be a jawmuncher.

This word is commonly used in middle and high school when talking about a friend that does stupid things outside of school but is smart inside of school.

He's a Jawmuncher

Person 1: Hey are you hyped for the new Dino Crisis game?
Person2: No, I'm not a jawmuncher