The teen was pronounced dead Wednesday evening, Pine Bluff police said in a prepared statement.
Monday’s shooting happened at Watson Chapel Junior High in Pine Bluff, a city of about 43,000 people located about a 45-mile drive southeast of Little Rock.
A suspect — also a 15-year-old boy who was a student there — was taken into custody Monday after being found hiding behind a house near the school, police said.
A motive wasn’t immediately known to investigators, Pine Bluff Police Chief Kelvin Sergeant said on Monday.
The suspect was being held Monday in the juvenile justice center in Pine Bluff, authorities said. Formal charges had not been filed as of Monday.
The school was to reopen for classes Thursday, following two days of virtual instruction, the school system said Wednesday.