Boy, you wouldn't know how to write your name if it wasn't stenciled on your locker.
6 seconds sound clip from the Dazed and Confused (1993) movie soundboard.
You can hear this line at 00:11:28 in the Blu-ray version of the movie.
Quote context
[...]
- So, tell me. Any of you girls gonna be ready to play some football this fall?
- What do you think about that, huh?
- Boy, you wouldn't know how to write your name if it wasn't stenciled on your locker.
- What's the matter with you, Floyd?
[...]