Aaron Feis, who was also a security guard, suffered a gunshot wound after protecting students and died later in hospital, according to school spokeswoman Denise Lehtio.
Mr Feis, 37, was one of 17 people killed after a gunman opened fire at the Parkland high school on Wednesday, sending hundreds of children streaming out onto the streets in fear.
"He died the same way he lived - he put himself second," Lehtio told CNN.
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"He was a very kind soul, a very nice man. He died a hero."
Marjory Stoneman Douglas High's Twitter account also hailed its assistant coach as a "hero".
"It is with Great sadness that our Football Family has learned about the death of Aaron Feis. He was our Assistant Football Coach and security guard [sic]," it read.
"He selflessly shielded students from the shooter when he was shot. He died a hero and he will forever be in our hearts and memories."
Student Julien Decoste, who survived the gunfire by hiding in a closet with classmates, said he had to step over Mr Feis' body when fleeing the school.
"He shielded two kids from being shot. He took the bullets himself," Julian told NBC News.
"As I was being escorted out of the building, I had to step over him. Right then and there ... I knew: He had to have been dead or injured."
Sheriff Scott Israel told reporters on Thursday he knew coach Feis personally and said the man was adored by the community.
"I don't know when Aaron's funeral is, I don't know how many adults will go, but you'll get 2,000 kids there," Sheriff Israel told reporters. "The kids in this community loved him. They adored him. He was one of the greatest people I knew."
The Federal Bureau of Investigation was warned last year about an ominous online comment by the 19-year-old man accused of killing 17 people at his former high school but was unable to locate him, an agent said on Thursday.
Authorities said the ex-student, identified as Nikolas Cruz, walked into the Marjory Stoneman Douglas High School in Parkland, near Miami, on Wednesday and opened fire with an AR-15-style assault rifle.
He was charged with 17 counts of premeditated murder after being questioned for hours by police for the second-deadliest shooting at a public school in US history.
