The 17-year-old needed emergency surgery for a blood clot on his brain after the incident in Bingley's Myrtle Park.
He was among groups of young people celebrating their exam results on Tuesday at the park's riverside bottom meadow.
Det Supt Chris Thompson of West Yorkshire Police's Homicide and Major Enquiry Team, said: "He was subjected to a violent assault which left him fighting for his life with serious head injuries."
The teenager, from Allerton, Bradford, was in a "stable" condition at Leeds General Infirmary.
Police set up a cordon around the attack area yesterday.
Det Supt Thompson said: "Young people from Bingley Grammar School were present in the park at this time celebrating their GCSE results.
"I would appeal to this group of people, anyone who may have witnessed the incident or anyone who has any information, to contact the incident room. It is vital that we piece together the events leading to this incident."
The victim is not believed to attend Bingley Grammar School.
A 16-year-old boy was last night charged with wounding and resisting arrest and was due to appear at Bingley Magistrates' Court today.