The Quiet Kid's Test Score

Alright, listen up, you smooth-brained chucklehead. You see, this 'quiet kid' gets a bad grade, a 74, and then threatens to show the teacher their "own 74." The joke plays on the stereotype of the quiet kid as a potential school shooter. It's funny because instead of the grade, the kid implies they will show up with a gun, which is also an old rifle model, causing a panic. You thought it was funny but in fact you're a bit slow to get this joke, aren't you?