A hospital has confirmed patients from the school, and medical helicopters have been spotted at the school, CNN reported.
Santa Fe Independent School District posted on its website that the high school has been placed on lockdown.
One person is in custody Friday in connection with an active shooting at Santa Fe High School, according to the Galveston County Sheriff's Office.
The entire school district is on lockdown, district officials said. They're promising to release more information as soon as possible.
Leila Butler, a sophomore at the school, told ABC13, a local affiliate, that fire alarms at the school went off at about 7:45 a.m.
Galveston County Sheriff's Maj. In that case, students and teachers reported hearing "popping sounds" outside, as Houston Public Media reported. Douglas Hudson told the Associated Press units are responding to reports of shots fired but could not give further details.
A student at the school told the station she had been sitting in art class this morning when someone entered with a gun and began firing.
Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have also responded to the school.
Santa Fe is a city of about 13,000 residents, located 30 miles southeast of Houston. Other nearby school districts are putting protective measures in place as well.