The German Army committed one of its worst atrocities against civilians on the 20th October 1941. On the 19th they began rounding up thousands of men over the age of 16 in the Serbian town of Kragujevac. Amongst them were 300 boys taken out of the First Boys High School, together with 18 teachers.
On the 20th they were led out of the town and at least 2,300 were shot. The teachers were shot alongside their pupils.
The following day the Wehrmacht announced:
The cowardly and treacherous surprise attacks on German soldiers during the previous week, on which occasion 10 German soldiers were killed and 26 wounded, had to be punished. For that reason 100 people were shot for each killed German soldier, and for each wounded 50, mainly communists, bandits and their siders, 2300 altogether. Every similar case, even if it were only sabotage, will be dealt with the same severity.
Chief of Local Command
This was just one of the opening rounds, if one of the bloodiest, in the particularly cruel and bitter war within Yugoslavia.
Virtual Tourist has a summary of material in English.