The teenager was pronounced dead Wednesday night, Pine Bluff police said in a prepared statement.
Monday’s shooting took place at Watson Chapel Junior High in Pine Bluff, a town of about 43,000 people 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 on Monday after he was found behind a house near the school, police said.
A motive was not immediately known to the detectives, Kelvin Sergeant, police chief of Pine Bluff said Monday.
The suspect was held at the juvenile justice center in Pine Bluff on Monday, authorities said. No formal charges were filed Monday.
The school would reopen for classes on Thursday, after two days of virtual instruction, the school system said Wednesday.