Complex sentence: Principal Skinner then sends Bart a message to Edna making his classmates laughing at him making him furious and shows them what they are doing and tells them not tell their romance to someone especially Superintendent Chalmers.
Split into simpler sentences: Principal Skinner then sends Bart to relate a message to Edna in front of his classmates who laugh at him making him furious. He then shows them what they are doing and tells them not tell about their romance to anyone, especially Superintendent Chalmers.