Students and teachers were instructed by loudspeaker to lock their classrooms at around 11.15am local time (10.15am GMT) before police revealed it to be a "false alarm".
The official account of the press office of the Arnsberg district said: “The amok situation in Menden was a false alarm. Thanks to all the forces who acted fast!”
Helicopters were circling the area while sirens blared. Armed police rushed to the scene.
Some pupils contacted family and friends using mobile phones, with one describing the scene was "scary".
Police in Germany after a gunman entered a school in Menden, Germany
It is scary here, police officers are everywhere
They said: "It is scary here, police officers are everywhere, helicopters… For nearly an hour already."
Police said armed officers had been deployed to the neighbourhood as they hunt the man.
Armed police in Menden, Germany, after reports of a gunman
Investigators believed the man left the school, called Hönne-Berufskolleg, and was on the run on the streets of the town.
A Twitter user said: "Possible armed man in school in #Menden, #Germany. Teachers and pupils got order to lock doors. Heavy police presence on the scene."
A police helicopter circling the area following reports of a gunman
The school, which has more than 2000 students and 100 teachers, is located in Menden, near Dortmund in west Germany.
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