Two pupils from a Sutton school got into an argument and a 12-year-old boy was assaulted.
The incident, on Barnes Crescent, happened near Sutton Community Academy in High Pavement, on Thursday, July 18.
David Mackey, school headteacher, said: "We are aware of an incident that occurred in the community early yesterday evening.
"We have met with the parents of both involved children this morning.
"We have been informed that the Police are dealing with this incident and we will support where we can."
Inspector Craig Hall said: “We’re investigating a report of an assault on a 12-year-old boy, following a dispute with another boy, which happened on Thursday evening (July 18) on Barnes Crescent.
“The victim was injured and our enquiries are ongoing into the circumstances of what happened."