The teenager was pronounced dead on 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 located about 72 kilometers southeast of Little Rock.
A suspect – also a 15-year-old boy who was studying there – was taken into custody on Monday after being found hiding behind a house near the school, police said.
The reason was not immediately known by investigators, Pine Bluff Police Chief Kelvin Sergeant said on Monday.
The suspect was being held on Monday at the juvenile justice center in Pine Bluff, officials said. The formal charges had not been made until Monday.
The school is due to reopen for classes on Thursday after two days of virtual instruction, the school said on Wednesday.