Complex sentence: After this, Dr. Nelson calls all of the students to the Gymnasium to teach Cartman a lesson by making fun of his weight problem with the words "Hello Fatso!" when he walks in so he understands how it feels.
Split into simpler sentences: After this, Dr. Nelson calls all of the students to the Gymnasium to teach Cartman a lesson. He wants them all to yell, "Hello Fatso!" when he walks in so he understands how it feels.