Repudiation means the act of claiming that something is invalid. If there were a repudiation of all the math test scores because of widespread cheating, you'd probably have to take the test again.
Repudiation comes from the verb "repudiate," which is rooted in the Latin word repudiare, meaning to divorce or reject. If you show something is false, you have made a repudiation of that thing. Until the repudiation of the claims, people thought that heroin was a harmless ingredient that could help ease a cough. General belief in the safety of cocaine and cigarettes had similar repudiations.