Technically, The Wave has a happy ending: the experiment ends before too much damage is done, maybe some lessons are learned, and, well, nobody dies.
So let's talk about the two major things that go down at the end of the story:
(1) Ben's super-bizarre revealing of the experiment. Ben has decided that he wants the students to end The Wave on their own, but he basically forces it down their throats by comparing them to Nazis:
"Yes, you all would have made good Nazis […]. You would have put on the uniforms, turned your heads, and allowed your friends and neighbors to be persecuted and destroyed. You say it could never happen again, but looks how close you came." (17.38)
As one students says after it's all over, "What a freak-out" (17.44). Seriously: Ben's a little harsh, don't you think? Sure, The Wave members did act badly, but this is a far cry from anything approaching the Holocaust. Shmoop isn't sure Ben picked the best way to reveal things to his students. What about you? How would you have gone about it? (Not that you'd be in this situation to begin with!).
(2) Robert. Now this is a tough one. In less than a week, Robert has transformed from class-loser to Wave leader, only to have it all pulled out from under him in a most shocking way. He's clearly hurt, and given how quick he was to change initially, we have no idea what to expect now. So what do you think Ben and Robert will talk about? Is he going to be able to help this kid out? If you were Ben, what would you say to Robert?
One last thing: you know what frustrates Shmoop about this ending? We don't get to learn how this experience affects the people involved! We want to know what these students do after The Wave is dissolved. Are they scarred? Do they forget about it and move on? Well, that's up to the readers' imagination to decide.