A high school student wearing a “Born to Kill’’ T-shirt shouted, “Surprise!” — then opened fire on his school in Santa Fe, Texas, with a shotgun Friday morning, killing 10 students and a teacher and injuring 10 more, authorities and reports said.

“Someone walked into my class today with a shot gun. I heard 2-3 shots, and sh-t started flying at me,” student Zach Muehe wrote on Twitter.

“I can’t begin to explain the thoughts that we’re going through my head. I watched some girl shot dead right in front of me.”

Dimitrios Pagourtzis, a 17-year-old former member of the school’s football team who recently posted a photo of a gun on Instagram, was arrested after the 7:30 a.m. massacre, law enforcement sources told multiple outlets.

He was being held Friday afternoon on capital-murder charges, Galveston County Sheriff Henry Trochesset said.

A second “possible person of interest” was detained and is being questioned, Harris County Sheriff Ed Gonzalez said at a press conference earlier.

At least 10 others were injured in the shooting — including one law enforcement officer, according to the Houston Chronicle.

Texas Gov. Greg Abbott said investigators also found explosive devices, including a molotov cocktail, in the school and nearby.

One student told TV station KTRK that the gunman came into her first-period art class and started shooting.

“We thought it was a fire drill at first but really, the teacher said, ‘Start running,’” the student told the television station.

Students told reporters that the shooter yelled “Surprise!” as he burst into the room.

Pagourtzis was dressed in a trench coat and a shirt that said “Born to Kill,” witnesses said.

Another student at Santa Fe High School told The Post that Pagourtzis was odd and began to dress in a trench coat.

“He was kind of weird, I guess… he never seemed that right,” sophomore Alec Neal said.

“This year, what weirded me out was he started wearing a trench coat… I know earlier in the year he told me he was buying knives off of Amazon.”

The Columbine shooters infamously wore trench coats during the massacre at their Colorado high school.

President Trump spoke about the shooting before a press conference Friday.

“We grieve for the terrible loss of life and send our support and love to everyone affected by this absolutely horrific attack to the students, families, teachers and personnel at Santa Fe High,” he said.

“We’re with you in this tragic hour and we will be with you forever. My administration is determined to do everything in our power to protect our students, secure hands of those who pose a threat to themselves, and to others. Everyone must work together at every level of government to keep our children safe.”

With Post wires