The headteacher at Sandy Hook Elementary School was killed as she lunged at the gunman, according to reports.
Town officials said Dawn Hochsprung died when attempting to tackle the attacker after he forced his way into the school.
When asked whether Ms Hochsprung is a hero, the chairman of the town's legislative council, Jeff Capeci, said, "From what we know, it's hard to classify her as anything else."
Therapist Diane Day was in a meeting with Ms Hochsprung and the school psychologist Mary Sherlach at the Newtown school on Friday morning when the gunman began shooting at around 9.30am.
"We were there for about five minutes chatting, and we heard Pop! Pop!, Pop!" Ms Day told The Wall Street Journal.
"I went under the table."
But Ms Hochsprung and the psychologist jumped out of their seats and ran out of the room, Ms Day said.
Ms Hochsprung was confirmed to be among seven adults and 20 children killed during the rampage. Ms Sherlach was among those killed.
"They didn't think twice about confronting or seeing what was going on," Ms Day said.
Ms Hochsprung is also believed to have switched on a loudspeaker system in the school to alert students and staff to the danger.
"She was just an amazing woman... the kids loved her. It's a huge loss for our school and our town," said assistant librarian Maryanne Jacobs.
Fourth-grade teacher Theodore Varga said: "You could hear the hysteria that was going on.
"Whoever did that saved a lot of people. Everyone in the school was listening to the terror that was transpiring."
He added that a school caretaker also ran out warning people there was a gunman on the premises.
The masked gunman has been named as 20-year-old Adam Lanza.
Robert Licata, whose six-year-old son was in one of the classrooms when the gunman burst in and opened fire, said he told him the shooter did not say anything.
He said: "That's when my son grabbed a bunch of his friends and ran out the door.
"He was very brave. He waited for his friends."
First-grade teacher Kaitlin Roig barricaded her 15 students inside a tiny bathroom with a bookshelf and told them to be "absolutely quiet".
She told ABC News: "I said, 'there are bad guys out there now. We need to wait for the good guys'.
"They asked, 'Can we go see if anyone is out there? I just want Christmas. I don't want to die, I just want to have Christmas.'"