Callum McVeigh was reported missing on Friday to South Yorkshire Police.
Callum, who was living in Rotherham, but is originally from Huddersfield, was last seen leaving for school around 8.15am yesterday.
West Yorkshire Police have been looking for the youngster, who is thought to be in the Huddersfield area, since yesterday afternoon.
Callum, who is 5ft 5ins and has black hair, was last seen wearing a black school blazer, black school trousers, black shoes and carrying a black backpack.
Detective Inspector Mark Catney of Kirklees District CID, said: “We are carrying out extensive enquiries including extra patrolling to find Callum and have reason to believe he is in the Huddersfield area.
“Callum is only 13 and may be in need of treatment for a medical condition, so we are obviously concerned for his well-being and need to find him."