The boy is in critical condition in hospital and another boy, aged 16, has been arrested on suspicion of attempted murder and remains in custody.
The stabbing happened in a field near Marches Road, Warnham, near Horsham, in the early hours of this morning.
Sussex Police said they understood there were “many young people in attendance” at the time of the stabbing.
Two stretch tents and a gazebo were set up in the field, which had been mown for the gathering.
Two forensics officers were also next to a black tent further away from the cordon.
Chief Superintendent Jo Banks said: “We are urging witnesses and anyone with information to come forward.
“We understand that there were many young people in attendance at this address at the time who will know important information to help the investigation.
“In particular we would also ask parents and carers of those who attended to speak with their children and to contact us with information.
“This is a fast-moving and complex investigation.
“Officers will be working in partnership with local schools and there will also be an increased police presence in the area in order to provide reassurance.
“We believe this was an isolated incident that does not pose a wider threat to the community.”
Witnesses or anyone with information can report it to Sussex Police on 01273 470101 or online and quote Operation Ketley.