One of the points of this story is the intended demonstration of the idea that we cannot learn from the past unless we remember and understand the past. Though we may believe that terrible incidents and behaviors of human history cannot be repeated (ex: what happened with slavery in the U.S. and the terrors of Nazi Germany).
The story here shows us that these behaviors are human, not purely historical. These behaviors grew of out human impulses that do not die away on their own.
It is the nature of the human condition that causes power to be a corrupting influence.
Past horrors can return if the past is not understood and remembered consciously and conscientiously.
The Wave becomes a potent example of human behavior getting out of control and re-enacting a well-documented and rather evil set of psychological traits. The students felt they would never succumb to such a psychology. They knew too much about history to do so. This, clearly, is not the case.
Another lesson here relates to individual strength and the difficulties of overcoming group-thinking within the context of the group.
It is much easier not to have to think...
The students who strive to force the people in the Wave to look at themselves for what they have become find extreme difficulty and danger in doing so. Standing against a unified group, the individual is forced to call upon great strength and integrity and spirit, yet even these may fail to change the course of the group's behavior.