The victim suffered a knife wound and has been taken to hospital for urgent treatment after the incident on Harehills Lane.
Police attended the scene of the assault at 3.42pm today.
Another boy, 15, has now been detained in connection with the incident.
Harehills Lane is shut between Compton Road and Ashley Road due to the incident. Enquiries are ongoing.
Buses are being diverted via Stanley Road. Services affected are the 42 and 91.