A Houston mother, who said her daughter was well-behaved at home, was worried about what was going on in her child's classroom because the girl had been suspended four times for bad behavior.
So, Diana Mijares decided to secretly bug her daughter's backpack and was shocked to hear what was on the tape.
"It made us concerned," Mijares said on "Good Morning America" today. "It was enough and we needed answers."
Megan Mijares' digital tape recorded mostly mundane moments at Memorial Elementary School's prekindergarten class, but then it captured the teacher yelling at the group of 4- and 5-year-olds. All of it happened without Megan's or her teacher's knowledge.
"You're just a bad kid," the teacher says on the six-hour tape. "You're mean to me, so I get to be mean to you."
The teacher, who was not identified, continues to harshly scold the children.
"You are all just stupid kids. I swear to God," the teacher says. "You are just all stupid kids."
Mijares said at that point the teacher was responding to a boy who was moving slowly.
"I believe one of the students was taking a little bit longer to come into the classroom," Mijares said.
In response to the allegations, the Houston Independent School District is investigating the case and the school's principal has reassigned the teacher.
Mijares worries that action will not be enough.
"My fear is that if she's reassigned that maybe possibly this could happen to other children," said Mijares, who added she only was trying to protect the children.
"They are learning bad manners and they don't need this," Mijares said.
The teacher has a history of parental complaints. Another mother told ABC News Houston affiliate KTRK that the teacher yelled at her young son and then slapped his face in December.
Houston school officials confirmed a slapping allegation was reported to the principal. After an investigation, the principal found evidence the teacher slapped a student and she was disciplined.
"She was suspended for one day without pay back in December," said district spokesperson Terry Abbott.
Mijares said it's important for parents to pay attention and be involved with their children's schools.
"My advice is if you feel something is really going on and you can't get any answers from the school, try to take charge as much as you can," she said.
Now she and her husband, Oscar, are trying help Megan recover. She said that the girl still is having some problems in school and that they worry the teacher's harsh words may have affected the girl.
One parenting expert said such words could be damaging to young children.
"Four-year-old children see themselves as stupid when you call them stupid. They are going to act up if you call them mean and bad because that is how you see them," said Nancy Samalin. "They are not going to disappoint you."
The teacher's attorney told ABC News the 30-year veteran "wishes to apologize to the students and the students' parents. Her actions that day were an aberration … the result of medication that she was taking as a result of a recent hospital visit."