Images from the scene show the extent of the police presence, with students lined up outside the premises.
Santa Fe Independent School District (SFISD) confirmed in a Facebook post that there have been injuries but did not provide detail on how many were wounded or how severely. SFISD also said that the situation "is active, but has been contained."
Speaking to reporters on the scene, Assistant Principal Dr Cris Richardson confirmed that one person had been arrested and is in custody. However, she could not confirm any casualties.
Galveston County Sheriff Ed Gonzalez and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) have confirmed that their officers are responding to the incident.
There has been no official confirmation of injuries in the incident, but one local media outlet is reporting that at least one person has been shot in the leg.
Speaking to ABC 13, student Leila Butler said that at 7:45am the school fire alarms went off and students fled their classrooms. She says that some fellow students reported hearing shots.
Another witness, Paige Curry, told reporters that she heard four to five shots before the fire alarm went off, but could not confirm seeing any injuries.
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Comment: UPDATE from RT:
Up to 10 people have been killed at a school shooting in Santa Fe, Texas. Police confirmed that one person is in custody and a second person has been detained. A police officer is among those injured.
Speaking to reporters at the scene, a police spokesman said there had been between eight and 10 fatalities, including students and school staff. A male student has been confirmed as the shooter.