According to initial findings, the investigators do not believe that the suspect went on a rampage at the school in Lünen.
“I think we can rule that out," the investigating prosecutor Heiko Artkämper told the German Press Agency (DPA).
The background of the crime at the Käthe-Kollwitz-Gesamtschule, which is attended by nearly 1,000 pupils, was initially unclear.
“We are not yet able to give any information on the type of injuries the victim sustained," police stated. Authorities have however confirmed that the victim was 14 years old.
According to the police, the crime happened shortly after 8am on Tuesday morning. Both the victim and the suspect are believed to have been pupils at the school. Police added that they were unable to give details of the wounds that caused the victim's death.
A homicide squad has taken over the investigation and are set to question the suspect.
“At the moment, investigations are in full swing to find out why this happened," said a police spokeswoman. Officers at the crime scene were searching for evidence and questioning witnesses early on Tuesday afternoon.
German media said both pupils were German citizens, with the suspect also holding Kazakh nationality.
According to Bild daily, the suspect stabbed the victim in the neck with a knife in front of his mother.
A minute's silence will be held on Wednesday at midday in the school and at the city hall of Lünen.
The Kollwitz-Schule is one of two comprehensive schools in Lünen, a town on the border between the Ruhr area and Münsterland.
Recently, state politicians have been campaigning against juvenile criminality in NRW. Former North Rhine-Westphalian Interior Minister Ralf Jäger (SPD) described it as "one of the most important tasks facing the NRW police force".
In 2016, one in five criminals in the western state was younger than 21 years of age.