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. 

What major life lesson do Laurie, David, and Mr. Ross learn in "The Wave"?

In "The Wave" by Todd Strasser, a teacher, Mr. Ross comes up with an idea to help his students learn about the Holocaust. He creates a fictional group called the Wave where students are expected to conform to the ideals of the group for the "good" of the group. Unfortunately, his plan gets out of hand when the students learn the lesson Mr. Ross is trying to teach too well. They begin to blindly follow the imaginary leader of the Wave and ostracize students who refuse to join them, eventually physically bullying those who will not. When Laurie writes an article about the negative impact of the Wave, her boyfriend, David, tries to convince her to stop spreading her views. In the midst of an argument, David shoves Laurie, and she falls to the ground. David realizes that she is right about the Wave, and they go to Mr. Ross' house to speak to him about ending it. When Mr. Ross sees that they are right, he calls the students together in an assembly to meet their supposed leader. The movie screen is set up, and as Mr. Ross introduces the Wave's leader, they all see Adolf Hitler's image on the screen. In the end, Laurie, David, and Mr. Ross understand that their individuality is important and that no one should blindly follow any leader. Their big lesson is that even though it may be easier to be a part of a group, it is more important to think for themselves and to  question the group's motivation and activities, instead of accepting everything someone tells them as fact.