What is the life lesson learned in the novel The Wave by Todd Strasser?

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.