According to prosecutor Ola Björstrand, the 17-year-old lied in interrogation when he said that he was the one who shot the man at Långbergsskolan to death.
A 24-year-old man has been convicted of the murder in both the district court and the court of appeal, but appealed the verdict to the Supreme Court.
The 17-year-old suspect is now in custody.
The trial against him begins in Nyköping District Court on 6 December.
The text is updated.