The victim was walking home from school along Sycamore Avenue, Halton, with a friend when he was approached by a man who demanded his mobile phone.
When he refused the suspect punched him twice to the head.
The victim suffered a broken jaw that required pinning in two places. The incident happened between 2.50pm and 3.20pm on Wednesday, January 11, but police only released details today.
The suspect then made off towards Primrose Valley. He was described as mixed race, 6ft tall, aged in his early twenties and of scrawny build.
He was wearing black clothing with a black baseball cap and had his hood and cap covering his face. He spoke with a strong Leeds accent.
Detective Constable Matt Cooper, of Leeds District Crime Team, said: "This was a violent attack on a young victim that left him with serious injuries that required significant hospital treatment. He has been understandably traumatised by what happened and we are carrying out enquiries to identify the person responsible.
"We would like to hear from anyone who saw a man fitting the description in the area or who has any information that could assist the investigation."
Anyone with information is asked to contact DC 543 Cooper at Elland Road via 101 quoting crime number 13170016589 or call the independent charity Crimestoppers anonymously on 0800 555 111.