The attack happened between 9am and 11am on Sunday in the playing fields of King James’s School, Knaresborough, when the victim, a local teenage girl, was grabbed by a man.
As well as issuing a description of the suspect, police are appealing for two potential witnesses to come forward.
The suspect is described as 5ft 7ins to 6ft and of large build - not fat, but muscular and heavy-boned. He wore baggy black trousers and an oversized black hoodie with a large pocket to the front, and his face was covered.
Detectives also need to speak to two separate men who were seen in the area. One was seen in the school grounds between 7am and 7.15am, the second between 9am and 9.15am.
Detective Chief Inspector Allan Harder, of North Yorkshire Police, said: “It is very important that I speak to anyone who was in the area of King James’s School on the morning of Sunday 5 April.
“If you have not already contacted the police, please do so straight away.
“In particular, I want to trace two separate men who were seen in the school grounds on that day. I believe that both of these men may have important information that could assist this investigation.
“I would also urge anyone who recognises the description of the suspect and his clothing to contact me as a matter of urgency.”
Witnesses should call North Yorkshire Police on 101.