Explain why the rally is the climax of the story?

The rally near the end of the novel is climactic because the students learn that all of the "propaganda" associated with the Wave and all of its slogans, behavior, and treatment of other students not in the Wave is exactly like what happened in Nazi Germany during World War II.  The learning of the students is that it is quite easy to be caught up into such a horrible set of beliefs and that no one is immune from such teaching.  The Wave quickly disbands; Robert, the one student who really profited from the behavior of the Wave, is now alone because he has no position in the normal teenage heirarchy.