This is the latest in a series of mass shootings in the US in recent years.
- Copyright: BBC
Police are searching a mobile home located a few miles from the campus for possible explosives, according to local news reports.
The US-wide National School Walkout campaign group points out that students at Santa Fe took part last month:
The youth-led movement for gun control, created in the wake of the school shooting in Parkland, Florida, tweeted "we are with you".
The group noted in its statement that Friday's shooting is the deadliest since the February shooting in Florida.
Local reporter Jake Reiner says "bomb techs" are surrounding a house 3 miles (5km) from the school.
Brandy Bensassi, whose daughter attends a school in the same district as Santa Fe, has told the BBC that numerous schools are in lockdown.
"The whole district goes on lockdown no matter how close they are to the shooting as a precaution," she said, adding that "police are everywhere."
"All I know about the attack is a boy saw the shooter walk in with a shotgun and he pulled the fire alarm to alert everyone."
They are still on lockdown until the police issue a report telling everyone it's OK to resume as usual," she said.
More from President Trump (see 16:49 entry).
He says: "We grieve for the terrible loss of life and send our support and love to everyone affected by this absolutely horrific attack.
"My administration is determined to do everything in our power to protect our students, secure our schools and to keep weapons out of the hands of those who pose a threat to themselves and to others.
"May God heal the injured and may God comfort the wounded and may God be with the victims and with the victims' families. A very sad day. Very, very sad."
The Sante Fe High School is about 40 miles (65km) south of Houston, Texas.Copyright: BBC
Officials have said at least nine people are being treated at area hospitals.
Several students have spoken to local media and described hearing a fire alarm go off shortly before 08:00 local time.
It is not clear how the alarm was activated.
One witness told KTRK-TV the shooting happened in her art class.
"There was someone that walked in with a shotgun and started shooting," the student said, "and this girl got shot in the leg."
Texas police say first reports of an "active shooter situation" emerged just before 08:00 local time (13:00 GMT).
They say one person - a suspect - is in custody.
A second possible person of interest has been detained and is being questioned.
The shooting comes three months after 17 people were killed at a school shooting in Parkland, Florida. Students at Marjory Stoneman Douglas High School expressed their grief on Twitter.
Harris County Sheriff Ed Gonzalez told reporters that preliminary reports show "eight to 10 fatalities" including students and adult members of staff.
An officer was also injured.
At a news briefing, President Trump condemned the shooting.
Referring to the victims' relatives and friends, he said: "We're with you at this tragic hour.
"Everyone must work together at every level of the government to keep children safe".
One of the students describes how he fled campus after a gunman had fired shots inside the school.
He says he saw "a girl that got shot".
US President Donald Trump has tweeted about the attack.
First Lady Melania Trump has also reacted, tweeting that her "heart goes out to Santa Fe".
Up to eight people have been reported dead in a shooting at a Texas high school. Here’s what we know so far:
- Officials say one person is in custody after an attack at Santa Fe High School, which is about 40 miles (65km) south of Houston
- The school district has confirmed people were injured in an "active shooter" incident as classes began on Friday
- The sheriff of Harris County, the largest county in Texas, tweeted that his officers were responding to a "multiple-casualty incident".
We will be bringing you all the latest updates on this developing story.