According to Klein ISD, the student was detained and expelled on Friday morning after bringing the weapon to school.
The FBI says the plot involved pipe bombs and guns. The case spans two counties, with the girls attending two different Houston-area school districts.
A 16-year-old girl was shot in a car with three men and now one of those men is facing a tampering charge, but Houston police still haven't said who killed her.
Klein ISD says the gun belonged to the now-expelled student's parent, who could face legal consequences for allowing a minor access to a gun.
Police are on the hunt for man who allegedly sold a gun to a Deer Park High School student who then brought it to school.
A student was arrested after a tip led to a gun being found inside the halls of Deer Park High School. Parents grow concerned, saying these kinds of scares are unfortunately commonplace for children.