Jordyn Howe, 15, is accused of killing fellow teen Lourdes Guzman-DeJesus on the way to school in November, The Miami Herald reported.
Police said Howe pulled a .40-caliber pistol out of his backpack on the school bus. The gun went off and a bullet struck Guzman-DeJesus in the neck.
On Wednesday, Miami-Dade Circuit Judge Orlando Prescott ruled the case would be moved into adult court despite Howe's age.
Howe will be formally charged on Jan. 28 with manslaughter, carrying a concealed firearm and possession of a firearm by a minor.
Ady Guzman-DeJesus, Lourdes' mother, said she was satisfied with the judge's decision.
A juvenile detention center is "not where this young man belongs," her lawyer, Ron Book, said. "This is a 15-year-old man who knew exactly what he was doing with a handgun."