It happened outside Hildebrandt Intermediate School in the parking lot adjacent to the high school Wednesday.
A 17-year-old was driving a red Mini-Cooper in the lot while another 17-year-old was riding on top of the vehicle, according to Harris County Sheriff Ed Gonzalez.
Investigators on the scene said the driver suddenly applied the brakes, which caused the victim to fall off and hit his head on the pavement.
The victim was airlifted to a hospital, according to Justin Elbert with Klein ISD. Gonzales said the boy suffered a serious head injury.
The crash is under investigation of the Harris County Sheriff's Office Vehicular Crimes Division.