To discredit is defined as to cast doubt on the truth of something or someone, or to damage a person's reputation.(verb)
When you tell everyone that your professor knows nothing and show evidence to prove it, this is an example of a time where you discredit your professor.
See discredit in Webster's New World College Dictionary
See discredit in American Heritage Dictionary 4
transitive verb dis·cred·it·ed, dis·cred·it·ing, dis·cred·its
Learn more about discredit