A teenager was stabbed and two schoolboys injured during a large-scale disturbance at a park in Clarkston, East Renfrewshire.
It is understood the 16-year-old had his chest punctured with a screwdriver and was slashed on the hand with a saw.
One of the other victims, who are both 16, suffered a head injury while the third sustained minor injuries.
More than 100 pupils are thought to have gathered in Overlee Park to celebrate their exam results.
It is understood they were attacked by another group of youths who got out of a van.
The injured teenagers were taken to the Queen Elizabeth University Hospital in Glasgow.
The stab victim remains in hospital where staff describe his condition as stable.
Police said the two other teenagers have been released.
A large number of emergency services vehicles, including a helicopter, and ambulances were sent to an area between the park and Netheron Braes at about 20:15 on Tuesday.
Det Sgt Jim McLauchlan said: "We believe this to be an isolated incident involving two separate groups of teenagers in the area and extensive enquiries are ongoing to establish the full circumstances around what happened.
"Police are following a positive line of enquiry in relation to the incident and I would urge any members of the public who were in the park yesterday evening and witnessed a disturbance, or anything at all suspicious, to get in touch."
The force also confirmed there will be additional patrols in the area.
Insp Bryan McGeogh said: "I understand this incident will cause concern amongst the local community however I would like to reiterate that it is believed to be isolated."
Local independent councillor David Macdonald said problems with anti-social behaviour and violence in the park had got "out of control".
He told the BBC's Good Morning Scotland programme: "I think we need a much more pro-active, community-led policing strategy to be able to beat it.
"The visible police presence currently in this area is practically non-existent and the response to tackle the issue has, in my view, been lacklustre and muted at best.
"The offenders know this and they know they are getting away with it."