Charlie slipping the crumpled-up introduction to the poetry textbook into his mouth when the headmaster interrupts the textbook-ripping session. The look on his face is priceless.
For that matter, the entire first lesson with Mr Keating. Particularly when he tells the class to tear out the introduction to the poetry text. Reactions range from incredulous to gleeful.
Knox is at a party with a lot of drunk football players, and two of them assume he's the brother of one of their friends, "Mighty Mutt". He tells them he's not. They completely ignore him, pour him a drink and keep him there as they go on about what a great guy his "brother" is.
Charlie convincing Knox that the Society will help him win Chris (using what Keating had told them about the original DPS):
Charlie: "Women swoon", Knox!
Knox: But why do they swoon? (Charlie runs off and Knox takes off after him) Charlie, tell me why they swoon!
Charlie's prank — a phone call from God that he wants girls to go to the school as well — is rather stupid, but still funny.
After Charlie's prank of God "calling" the school to say girls should attend the school, Mr. Keating gently but firmly chides him for it. Then, as he leaves the room, he pauses momentarily:
Mr. Keating: Phone call from God? If it had been collect, that would have been daring.
Neil and Todd's jokes about the desk set that Todd's parents sent him on his birthday for the second time in a row. Eventually Neil notes how "aerodynamic" it is and, with Todd's permission, frisbee-throws it off the edge of the balcony.
Neil: I wouldn't worry about it... you'll get a new one next year.
Charlie deciding that from now on he wants to be called "Nwanda". Especially after he gets paddled for his prank, and Neil, calling him Charlie, asks what happened:
Charlie: Dammit, Neil... the name's Nwanda.
Charlie is made of funny. Other examples include "getting red"—unlike the modern meaning, it meant drawing a huge lightning bolt on his chest, for power and virility. And his Large Ham delivery of "TO INDEED BE A GOD" during the poetry and football scene.