An armed person was detained and taken into police custody after the shooting and the injured person was taken to hospital, police said.
"The students are in their classrooms with their teachers. The police have the situation on site under control," police added in a statement. "Police measures are continuing."
German paper Bild said the injured person was a woman.
Bild also reported that a second suspect appeared to be on the run. It earlier reported they were armed with a crossbow. Police said they were looking into whether more than one person was involved.
The shooting took place at the Lloyd Gymnasium, Bild reported.
Online newspaper Nord24 said a schoolgirl who heard shots had called the police. Students barricaded themselves in their classrooms, it added.