The 15-year-old male student was taken into custody at the scene in Homestead and later charged with manslaughter and carrying a concealed firearm. The Associated Press does not identify juvenile offenders.
An arrest report says the minor took a gun from his backpack and began displaying it on the bus. He then fired it once, hitting Lourdes Guzman. The 13-year-old girl was airlifted to a Miami hospital, where she died.
Eight other children, including Guzman's 7-year-old sister, were on the bus but were not harmed. Authorities took the children and the bus driver to a police station to be interviewed.
Guzman attended the charter middle school Palm Glades Preparatory Academy. Her sister went to nearby Summerville Advantage Academy.
Concerned parents gathered at the shooting site, which was cordoned off by crime scene tape as authorities combed the bus for evidence and gathered children's backpacks and belongings. The shooting happened about seven minutes away from the school.
Fabian Otero said he was relieved his son Christian, an eighth-grader at Palm Glades, rode to school with his wife. He said he and his wife have been shaken by the killing and are going to discuss enrolling their son in a different school or possibly homeschooling him.
"I was stunned! I was stunned!" he said. "That hits really close to home. That's just scary."
About a dozen parents picked up their children early, said Lynn Norman-Teck, a spokeswoman for the Florida Consortium of Public Charter Schools.
Many of the students had not heard about the shooting and the school did not make an announcement before classes were dismissed, but word started to trickle out as they walked outside to waiting school buses.
Thirteen-year-old Eric Carillo said he was in the same class as Guzman a few years ago.
"She was a good girl. She was fun," Carillo said.
Sarah Baer said she wouldn't let her daughter walk home from school because there are rough neighborhoods a few blocks away.
"I always take my daughter to and from school. This is so heartbreaking especially right before Thanksgiving," she said.
The school bus was not equipped with video surveillance equipment.
(Copyright 2012 by The Associated Press. All Rights Reserved.)