A 12-year-old boy was left in tears after allegedly being yanked to the ground by a relief teacher at a Christchurch school on Thursday.

The student's mother has since lodged a formal complaint as well as notifying police about the incident on Friday.

"You never in your wildest dreams think that something like this would happen," she told Fairfax.

The teacher was working as a casual day reliever at St Thomas of Canterbury College when the alleged incident happened.

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In a video, the teacher is seen standing over the boy, who is lying on the ground crying.

"Stop crying", the teacher tells him, telling other students to go "upstairs" as he walks away.

The boy's mother said her son was pulled to the ground by the handle of his backpack, which was ripped as a result.

He had bruises and scrapes on his knees and has also hit his head.

Principal Christine O'Neill told Fairfax the school had dealt with the incident immediately.

"We were taking statements from the students involved and the teacher so we could ascertain facts to ring the parents and inform them."

The teacher was no longer working at any schools before the Education Council made a decision.