They were taken to hospital to be treated for their injuries, the force added.
A suspect has been arrested, while another, who is armed with a crossbow, remains on the run after the incident at the Lloyd Gymnasium, the German daily Bild reported.
Students barricaded themselves in their classrooms, while a schoolgirl alerted the police about the shooting, according to the online newspaper Nord24.
The police said its investigations into the incident are ongoing. “The students are in their classrooms with their teachers. The police have the situation on site under control,” it added.
The shooting comes just months after a student opened fire in a lecture hall at Heidelberg University, killing one person and seriously injuring three others.