SSDD

http://news.bbc.co.uk/2/hi/europe/7936817.stm

Now how might this have been different if the teachers were armed? I don't know if they're allowed or not in Germany and don't have time to do the research.

However, given this article my hunch is they aren't allowed.
http://www.earthtimes.org/articles/show/187558,germany-tightens-gun-control-laws.html

In fact the reason the article describes the need for gun control sounds reminiscent of the reason behind Germany's 1928 gun control laws. See the post earlier today, Nazis and guns.