Officers checked students entering the school in Flensburg, just south of the Danish border, on Wednesday morning and detained the German teenager after finding knives in his backpack. The chat surfaced on Tuesday evening and the violence was threatened for Wednesday.
Police said the suspect admitted in questioning that he was responsible for the chat post and said he had wanted to use the knives against other people. They said in a statement that the motive appeared to be "personal problems," without elaborating. He was sent to a psychiatric clinic for young people.
Lessons were held as normal after the teenager was detained.