The police arrested several people in connection with a major police operation at a school in Dinslaken because of an alleged sighting of weapons.
A police spokesman said on Wednesday in Essen.
He did not give the exact number.
According to information from the Neue Ruhr Zeitung, the weapon was discovered in the school's bicycle cellar.
Two students are said to have been seen there by a school employee with a gun in their hands.
When they saw the school staff, the students fled.
The school management called the police around 1 p.m.
Some of the students and teachers are still in school.
There are strong police forces on site, including a special task force.
It is not yet clear what type of weapon it is.
A police spokeswoman had previously said: "We consider this to be a dangerous situation and are on site with numerous emergency services".
The spokeswoman rejected rumors of a rampage at the comprehensive school.
According to the police, no one was injured.