On the Cartoon Network video site, the gym teacher actually said "crap."
Dee Dee: (after hitting Dexter with a dodgeball) You're out!
Kid: Dexter, help me! Please help me! (a dodgeball, offscreen, hits him) AUGH! I'm...I'm hit! Good...bye...Dexter...UGH!
Dexter: If you need me, I will be in the science lab!
Gym Teacher: What's this crap?
Dexter: Who are you?
Gym Teacher: I'm your substitute coach!
Dexter: But, but, but...
Gym Teacher: Quiet!
Dexter: But my excuse?
Gym Teacher: What excuse? Now! Soon!
First appearance of the Dodgeball Robot.
Apparently, Dexter's family doctor is called Snobgrass, as signed on the ill-fated gym excuse.
This episode reveals the fact that Dexter hates gym class, and would rather be in the science lab.
This episode was rated #1 in the US by a marathon that announced #1 episodes in countries around the world, as voted on Cartoon Network's website.
The new gym teacher's reaction to Dexter's excuse note is "What's this crap?" Edited versions of this episode have him saying "What's this?"