Students playing below scattered when the body hit the sidewalk.
The boy's mother arrived at the scene a short time later. The teen's brother also attends Dalton.
The junior at Dalton had won academic awards and was involved in school activities, according to officials.
Dalton said in a statement that it was providing counseling and support services on site and to the larger school community.
"The Dalton School community is deeply saddened by today's tragedy, which involved the death of a beloved 11th grade student," the school said. "Our thoughts and sympathies go out to the family."
Dalton has 1,300 students in grades kindergarten through high school and was founded in 1919.
It is located on Manhattan's Upper East Side, near Central Park and the Guggenheim Museum.
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