What, finally, made the students realize that The Wave was just an experiment?

In Chapter 17, Mr. Ross plans a rally for the students involved in the Wave. He gives a speech, shows a graphic documentary about the concentration camps and tells his students that Hitler is their leader..... he also commends the qualities his experiment has exposed in terms of his students' acceptance, telling them that they would have made “good Nazis”. Ross' students are completely shocked by what they've been a part of and ashamed of their actions. They are now well aware that the Wave was just an experiment.