The situation is still active but has been “contained”, Santa Fe High School said.
At least one girl has been shot by a gunman at the school near Houston, witnesses said.
Students said the shooter entered a classroom and opened fire with what appeared to be a shotgun.
The school said there have been “confirmed injuries” but did not elaborate on numbers.
Local police and officers from the Bureau of Alcohol, Tobacco, Firearms and Explosives have been scrambled to the scene.
One student who was inside the school said the fire alarm was triggered then the shooting began after pupils had left their classrooms.
Another described scenes of panic as pupils began running after hearing the gunshots.
Footage taken from a news helicopter shows police ordering a group of students to empty their backpacks in a nearby school.
A second clip shows three helicopters landed in a nearby car park. Medics can be seen wheeling a stretcher towards one of the aircraft.
A statement issued by Santa Fe High School said: “This morning an incident occurred at the high school involving an active shooter.
“The situation is active, but has been contained. There have been confirmed injuries. Details will be released as we receive updated information.
“Law enforcement will continue to secure the building and initiate all emergency management protocols to release and move students to another location.
“All other campuses are operating under their regular schedules. The district will continue to keep you updated as information is available.
“Safety and communication are our top priorities.”
More to follow…