The answer to this question can be found at the very beginning of chapter 11. Chapter 10 finishes with Jess being told that his best friend is dead, and chapter 11 begins with Mr. Aarons trying to gently explain to Jess what happened. It's​ actually quite important that Mr. Aarons is the person doing this, because it shows that he really does understand and support Jess and his friendship with Leslie. Jess's dad explains that Leslie's body was found down in the creek. Jess flatly denies any possibility of drowning, because she was a strong swimmer. Mr. Aarons then explains that "they" think she must have hit her head on something on the way down after the rope broke. Readers are meant to assume that Leslie drowned in the river because she was unconscious; however, I do want to stress that we aren't given a confirmed cause of death. It isn't necessary for the purpose of the story. Leslie is dead, and that devastating fact remains the same no matter how she died.