The St. Mary's County Sheriff's Office said Friday that Jaelynn Willey died at 11:34 p.m. Thursday.
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Earlier Thursday night, Willey's family said Jaelynn would be taken off life support at University of Maryland Prince George's Hospital Center.
Jaelynn was shot in the head Tuesday morning at Great Mills High School by 17-year-old Austin Rollins. Police say the two had been in a relationship that recently ended.
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A 14-year-old student, Desmond Barnes, was also injured.
Rollins was killed in the attack. It remains unclear whether he committed suicide or was shot by a school resource officer who responded to the attack.
The sheriff's office said Jaelynn was surrounded by her family when she died.