Police were responding to reports of gunfire at a high school in Santa Fe, Texas on Friday and the school district said it was on lockdown because of an incident involving an active shooter.
Few details were immediately available. A spokesman for the police department in Santa Fe, about 30 miles (48 km) southeast of Houston, said officers were "still establishing the scene."
It was not clear whether anyone was hurt.
The Santa Fe School District said in a statement that it initiated a lockdown after an active shooter incident at the high school. It said it would send out more information as soon as it was available.
Houston-area media reported that the Harris County and Galveston County sheriff's offices were also sending deputies to the scene, and that a medical Life Flight was called to the school.
Aerial video outside the school broadcast on local television showed police escorting lines of students out of the building and then searching them for weapons as many police cars and at least two ambulances with lights flashing stood by.
Sophomore Leila Butler told the local ABC affiliate that fire alarms went off at about 7:45 a.m. local time and students left their classrooms. She said some students believe they heard shots fired, and that she was sheltering with other students and teachers near campus.
Another sophomore, named only as Nikki, told ABC13 that: "Someone had walked in with a shotgun and a girl got shot in her leg."
The latest reports of a possible shooting at a U.S. school underscored a national debate over gun control and gun rights that intensified after an assailant killed 17 students and staff members in February at high school in Parkland, Florida.