A teacher who slapped a boy in the face in "a complete kneejerk reaction" has been sacked by the school and censured by the Teachers Disciplinary Tribunal.

Annmarie Catherine Whelch, who had been a relief teacher at the intermediate school for only eight days, hit a Year 7 student across the face with an exercise book after she saw him hit another student with the book in June last year.

Later, the boy asked Whelch to come and speak to him at his desk and then hit her on the side of her face.

"This was planned, as all the boys at the table began to laugh," Whelch told the tribunal.

She then told the boy in front of the class: "I understand why you did this to me and I realise I shouldn't have done it to you."

She told the tribunal that she had been at a hospital accident and emergency clinic until the early hours of that morning with her daughter, who was in severe pain.

"Tiredness, I feel, just tipped me over the edge," she said.

"It is still not an excuse as I should have known better than to strike [him]. I did not plan to strike him. I was shocked at my reaction ... It was a complete kneejerk reaction."

Whelch did not report the incident but the boy disclosed it a week later and Whelch confirmed that it happened. The school terminated its contract with her immediately and reported the incident to police and Oranga Tamariki.

Police gave her a formal warning for assault on a child.

The tribunal, chaired by Wellington lawyer Theo Baker, found that slapping the boy constituted assault, physical abuse and serious misconduct which brought the teaching profession into disrepute.

The tribunal has censured her and required her to do a course on anger management or classroom management skills, have a mentor who will have to report to the tribunal every three months for a year, and show the tribunal's decision to every school she teaches at for the next two years.

However, it decided to let her continue teaching because she "acknowledged her wrongdoing from the outset", was tired because of her daughter's pain, had no previous disciplinary history, was willing to attend an appropriate course, offered to apologise to the student and his family, and "has continued relieving at another school with no further incidents".

Whelch told the tribunal that she had experienced difficulties in her role at the school, "including some students with challenging behaviours".

"Having to go to a police station was just a horrifying ordeal," she said.

"I had never broken the law and was petrified at my interview. I felt humiliated and very ashamed of my actions."

She said in hindsight she wished she had never gone to that school.

"But I can't change time, I can only learn from my mistake and move forward," she said.

"Fulltime teaching is not an option for me, I am contented with day-to-day relieving.

"This incident has certainly heightened my awareness. I am more than happy to attend an anger management or class management session and would also like to apologise to the parents."

The name and location of the school were suppressed because of damage the school had already suffered from "the very serious criminal activity of a former staff member".