A teacher at the Kaltenkirchen school centre – which includes elementary, middle and special-needs schools – raised alarm when two girls told him that the boy was planning acts of violence, municipality spokesperson Sascha Stanelle said.
News agency DDP reported that the boy had also called a threat into the school earlier on Wednesday.
Police took the situtation seriously, evacuating some 500 students with buses midmorning while armed officers guarded school doors. Most students were taken to another school, but those who travel from the countryside to attend school were brought home.
The boy was arrested after a short police search in the nearby community of Elmshorn.
“There was no resistance and there were no weapons in play,” police spokeswoman Sabine Zurlo said.
Germany is particularly sensitive to school violence after 17-year-old Tim Kretschmer killed 15 people at his former school with his father's gun in the southwestern German town of Winnenden in March. He later killed himself during a shootout with police.
The massacre stirred up debate about stronger whether the country needs stronger gun laws or a ban on violent video games.