Improbability of the ending
While I generally loved both the original movie (The Wave) and saw this one as soon as I had a chance, I've come to realize one thing as I've matured and grown older. When something has as many followers as Rainer has at the end and tries to bring it to an abrupt end like he does - the momentum is still there.
I've seen similar "group psychology" events, such as fans at a football game and they don't react well to this forceful "break of momentum". Dennis stands up and defends the Wave at the end of the movie, and to be honest, I think most people would've joined HIM right then and there. Rainer had them driven into a powerful emotional surge - one which has an easier time continuing in the same direction rather than breaking apart.
Now, the movie has a definitive point to drive home amongst the viewers and thus we suspend our disbelief for this reason - but I know that had these events played out as they did in the movie, Rainer would've been pushed aside and the movement continued. The teens preached amongst themselves - not about Rainer, but about Die Welle. They bonded, they fought, they did things TOGETHER. Out of what Die Welle preached, they took in Community (Gemeinschaft) the most - and that is a VERY hard thing to break.