The law enforcement source said the suspected shooter was in custody after the attack at the school about 30 miles (48 km) southeast of Houston. Local media reported at least eight people were killed.
The sheriff's office for nearby Harris County said its deputies were assisting with a "multiple-casualty incident."
"This is no longer an active shooting situation and the injured are being treated," the sheriff's department said on Twitter.
Harris County Sheriff Ed Gonzalez said on Twitter that one person was in custody and a second was detained. "An injured police officer is being treated, the extent of his injuries are unknown," Gonzalez said.
U.S. President Donald Trump wrote on Twitter: "Early reports not looking good. God bless all!"
The latest shooting at a U.S. school underscored a national debate over gun control and gun rights that has intensified after an assailant killed 17 students and staff on Feb. 14 at Marjory Stoneman Douglas High School in Parkland, Florida.
Aerial video outside the Santa Fe 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, Dakota Shrader, told Fox 26 TV her 17-year-old girlfriend told her by phone that she was wounded but was recovering in a hospital. "My friend got injured," said an emotional Shrader. "Her leg, she got shot in the leg."
Dr. David Marshall, chief nursing officer at the University of Texas Medical Branch, said that the hospital was treating at least three patients - two adults and one person under 18. He said it was not immediately clear if that child was a student.
"I know that they have gunshot wounds but that's the extent of the knowledge I have at this point," Marshall told reporters at a news conference. "One adult is in our operating room."
"We're ready for more patients," he said. "We have not heard of any en route at this point."
In February, Santa Fe High School was placed on lockdown while police investigated a "popping sound" that was feared to be gunshots, but no threat was found, the school district said.
That was a false alarm, but school shootings have happened regularly in the United States since the Parkland massacre.
Last Friday morning a 14-year-old boy shot and wounded a student at a Southern California high school before fleeing the scene and being arrested, police said.
Then on Wednesday a police officer assigned to an Illinois high school shot and wounded a 19-year-old former student who had brought a gun to the school, authorities said.
