A 14-year-old boy was kicked in the stomach and punched in the face by a group of teenagers near Stantonbury Campus on Friday.
He was walking with friends in the underpass under the V7, from Linford Wood into Heelands at about 3.15pm when the gang blocked their path.
The offender attacked the victim before runnning from the scene.
He is black, about 5ft 6ins tall, aged about 14-years-old, of broad build, has neat hair and visible cheek bones.
The victim sustained a fractured finger and minor bruising to his face. He was treated at Milton Keynes General Hospital but has since been discharged.
Investigating officer, PC Adam Jones, from Milton Keynes police station, said: “The incident occurred during school closing time, so it is likely there could have been a number of witnesses to the incident.
“I would appeal to anyone with any information about the incident to contact me via the Thames Valley Police Enquiry Centre on 101.
“Alternatively, if you don’t feel comfortable speaking to the police you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at www.crimestoppers-uk.org.
“No personal details are taken, information is not traced or recorded and you will not go to court.”