It happened between 3.15pm and 3.30pm on Wednesday (February 3) when the youth was close to the tennis courts in the park, police said today (Saturday).
Police said he was approached by an unknown offender, who possibly walked from the direction of Park Street, and then spoke to him.
The offender hit the victim and left the scene.
The boy suffered facial injuries. He attended Stoke Mandeville Hospital and has since been discharged.
The offender is a black male, aged approximately 16 to 18 years old, 5ft 10ins to 5ft 11ins, and of muscular build.
He was wearing a black zip-up hooded top and blue jeans.
Police are treating the incident as an assault occasioning actual bodily harm.
Investigating officer PC Lucy Rattenberry, based at Aylesbury police station, said: “I would like to speak to anyone who witnessed this incident or has any information about it.
“This occurred shortly after school finishing time and in a public place so there potentially would have been people around who may have seen something which could assist the investigation.”
If you have any information contact the 24-hour police enquiry centre on 101.
You can also call Crimestoppers anonymously on 0800 555 111.