A white city public school teacher — who claims she was wrongfully fired over a controversial lesson on slavery in which she was accused of having black students lie on the floor while she stepped on their backs — plans to sue the city for $1 billion.

Patricia Cummings, 37, was canned in October after complaints about the highly unorthodox teaching method that left some students feeling humiliated and singled out by the exercise.

But Cummings insisted Thursday that she initiated the lesson in her class at MS 118 in the Bronx in good faith — and only one student and her parent objected.

Her sacking — and the torrent of criticism that followed — has rendered her permanently unemployable, Cummings said at a press conference alongside her lawyer, Thomas Liotti. “I have no career at this point,” she said.

She filed a $120 million lawsuit in Suffolk County, where she lives, against a slew of defendants ranging from the city Department of Education to Mayor Bill de Blasio to media outlets. But Liotti said they plan to build a $1 billion class-action case with other teachers claiming similar forms of discrimination.

Cummings said she was initially cleared after an internal school investigation but that the case was revived after media reports on the incident.

The teacher said her suit will be supported by the testimony of a black teacher at the school who witnessed the lesson.

That colleague found it effective — and said he would not have objected if his own kids had taken part.

“That 20 seconds of a teachable moment changed my life,” she said, adding that she was pelted with emails calling for her murder and was widely vilified as a racist.

“Anyone who has met me knows I don’t have that bone in my body,” she said. “I was brought up — you treat everybody the way you want to be treated.”

Liotta argued Thursday that the lesson only drew objections because of his client’s race.

“How does the City of New York expect to attract effective teachers when people like Patricia Cummings have to go through this kind of abuse?” he asked. “It’s outrageous.”

The Education Department initially reassigned Cummings before firing her after an investigation, according to agency spokesman Doug Cohen.

Their probe found that she used poor judgment but did not substantiate the corporal punishment charge.

“Ms. Cummings was terminated based on a thorough investigation and a review of her performance as an educator,” Cohen said Thursday. “We’ll review the complaint.”