Video of the arrest sparked widespread outrage and questions about what role police should play in schools.
Richland County Sheriff Leon Lott suspended Deputy Ben Fields after the incident, and fired him Wednesday.
"We believe that Mr. Fields' actions were justified and lawful throughout the circumstances of which he was confronted during this incident. To that extent we believe that Mr. Fields' actions were carried out professionally and that he was performing his job duties within the legal threshold," Fields' attorney, Scott Hayes, said in a statement.
Federal investigators have gotten involved. Another student arrested from the same Spring Valley High School classroom has spoken out. And the sheriff is criticizing a South Carolina law that he says muddles the role of school resource officers.
Sheriff says student deserves part of blame
Richland County has 87 school resource officers, Lott said, and the "actions by Deputy Fields was not typical of the job I expect them to do."
Fields did wrong and was fired for his actions, Lott said.
But the student must shoulder responsibility for the disruptive behavior that escalated to the officer being called in, the sheriff said.
"When a classroom is disrupted by a student, that disrupts the education process and the students can't learn and the teachers can't teach," Lott said. "We have to have discipline in our schools."
What the student did does not justify the officer's actions, the sheriff said, but she must take responsibility for her role.
"We must not lose sight that this whole incident started by this student," he said. "She is responsible for initiating this action. Some responsibility falls on her."
Sheriff: The student hit the officer
At least three videos have surfaced of the violent arrest at Spring Valley High School. The sheriff said one of the videos shows the girl attacking the officer before the arrest.
"When the officer puts his hands on her initially, she reaches up and she pops the officer with her fist," he said.
Still, after watching all the footage, Lott told reporters that he "wanted to throw up."
"There's no justification for some of his actions," the sheriff told CNN's "Anderson Cooper 360°" on Tuesday night.
The videos show the officer standing over the student, seated at her desk. He puts his arm near her neck, then yanks her backward. The desk, which is attached to her chair, tips over; the student crashes backward onto the floor.
But Fields didn't let go, lifting her slightly off the ground. She flies out of her desk and slides several feet across the floor.
"He regrets it," Lott told CNN about Fields on Wednesday. "He's been at that school for seven years. He loves those students. ... He wishes it never happened this way. This wasn't his intent."
Conflicting reports on injuries
Sheriff's department spokesman Lt. Curtis Wilson said there were no reports of any injuries. And the sheriff said he did not believe the girl was seriously hurt.
"To my knowledge, she wasn't injured whatsoever," Lott said. "She might have had a rug burn or something like that, but she was not injured."
But Todd Rutherford, the student's attorney, said his client now has to wear a cast on her arm.
The girl also suffered a bruise on her head, her attorney said.
Student: It started with a cell phone
The 16-year-old girl who was taken to the ground was arrested on a charge called "disturbing schools." A classmate, Niya Kenny, 18, was also arrested on the same charge.
Kenny told CNN's Don Lemon on Tuesday night the incident started when her math teacher told the other girl to give up her cell phone. The girl refused and defied orders from the teacher and an administrator to leave the classroom.
That's when Fields
was called in, Kenny said. The school resource officer asked the girl to leave the classroom with him.
Kenny said the officer moved the girl's laptop off her desk.
"He grabbed her arm, and he put his arm around her neck at first. So that's why you actually see her -- if you get the right video -- then you'll see her trying to swing at him," Kenny said.
"And at that point, he just flipped the desk back and grabbed her out of it and threw her. And that's when you see her rolling across the floor."
Kenny said she and other classmates had their cell phones recording because of the officer's reputation.
"When he came in the classroom, I immediately told my classmates, 'Get your phones out, get your phones out. I think this is going to go downhill.' And it did."
Her attorney, Simone Martin, said she's been told "by a number of the students that he is referred to as Officer Slam as opposed to Officer Fields
. And that's telling."
Kenny was arrested and accused of disturbing school after yelling and cursing at the officer, according to an incident report.
Civil rights investigation
The FBI and U.S. attorney's office have opened a civil rights investigation to determine whether federal laws were violated during the student's arrest, a Justice Department representative said.
The sheriff said the FBI is also to be the lead agency in a criminal investigation.
"We do not want any issues with the community or those involved having questions concerning conflicts of interest in this investigation," he said.
Richland School District Two Superintendent Debbie Hamm thanked the sheriff and his department Wednesday for their "swift response."
"There may be some who think this is the end of the matter. In Richland Two, however, we continue to work closely with the Sheriff's Department and independent federal and state investigating agencies to examine exactly what happened," she said.
Analyst: Officer within his rights
CNN law enforcement analyst Harry Houck cautioned against jumping to conclusions about Fields
, even if the footage "looks really bad."
If an officer decides to make an arrest, he or she "can use whatever force is necessary," said Houck, a retired New York police detective.
"So if you don't comply with my wishes," he said, "then I can do whatever it takes to get you out of that seat and put handcuffs on you."
Sheriff criticizes law on disrupting school
Houck said the officer shouldn't have been called in to deal with the student in the first place.
"Too often, these teachers in these schools are calling on the cops because they have a disruptive student in the classroom," he said. "This is not a cop's job."
But South Carolina has a law that muddles the role of school resource officers, the sheriff said.
"Unfortunately, our Legislature passed a law that's called 'disturbing schools,' " he said.
"If a student disturbs school -- and that's a wide range of activities, 'disturbing schools' -- they can be arrested. Our goal has always been to see what we can do without arresting the kids. We don't need to arrest these students. We need to keep them in schools."