Concerned parents told The News the boy was believed to have been stabbed during an altercation near Priory School, in Fawcett Road, Southsea, about 3.30pm on Thursday.
Hampshire Constabulary said he suffered a ‘minor laceration’ to his abdomen and was taken to the Queen Alexandra Hospital, in Cosham, for observation.
Another 15-year-old boy, from Portsmouth, was arrested in connection with the incident. He has been released from custody but remains under investigation.
A police spokeswoman said investigations into the exact circumstances of the isolated incident and how the injury was caused were ongoing.
Neighbourhood police officers have been speaking to the school and are in the area to offer reassurance to anyone who has any concerns.
It is not known whether the two boys are students at the school.
A spokesperson for Priory School said: ‘The health and safety of our students is always our number one priority.
‘There was an isolated incident after school hours in a road near to the school on Thursday. There were no serious injuries, the individuals involved have been identified, and we are working with the police who are currently investigating.
‘With an ongoing police investigation, we cannot make any further comment at this time.’
People living near the area spoke of their shock and said this was not the first time something like that had happened.
Jack Smith, of nearby Manners Road, said: ‘I couldn’t believe this had happened when I was told.
‘It is awful and I hope the kid involved is okay. This isn’t the first time a youngster has attacked another one with a knife.
‘Luckily it does seem to be an isolated problem.’
A 48-year-old, who works along Fawcett Road, added: ‘It is shocking. There is a lot of crime in this area but it is usually petty crime.
‘It is horrible to think it’s two schoolchildren involved.’
Anyone with information is asked to call police on 101, quoting 44170356616, or Crimestoppers anonymously on 0800 555 111.