The woman was fired from Newberries Primary School in the village of Radlett, Hertfordshire, on Friday after parents were outraged after hearing about the remark.
She is said to have been teaching a class of 28 youngsters aged around 10 – of which 11 are Jewish – and said: “You better finish off your work quick, or I’ll ship you all off to the gas chambers.”
One of the pupils reportedly challenged her over the vile anti-Semitic remark, to which she said she was “joking” and apologised, while asking the kids not to tell anyone.
However, a number of pupils informed their parents who then lodged a complaint with the school.
An emergency meeting was convened by the board of governors that evening, with the teacher dismissed from her role the next day, according to the MailOnline.
It was thought the sacking was so swift because the woman was not on a permanent contract and was actually an agency worker.
One parent, who wished to remain anonymous, said: “To the credit of the school they made it clear they wanted to deal with the situation without delay.
“They did not seek to dispute the suggestion that the teacher had made the reference to ‘gas chambers’.
“The fact that the woman was sacked the following day says everything you need to know.”
A spokesman for the school commented: “We can confirm that last week we were made aware that an alleged racist comment had been made by an agency teacher working within the school.
“We acted on this immediately and our governors and leadership team are undertaking a full investigation. The agency teacher will not be returning.”
Following the incident, school bosses also sent home a newsletter saying it would be inviting leaders from different faith groups to talk to kids in a bid to “celebrate diversity”.
It added: “This year we are asking particularly for the focus to be on strengthening the important messages of religious tolerance and respect.”