Brenda Davies, 50, a teacher for 28 years, said she felt "betrayed" by her head, Graeme Russell, after the incident at Tennyson Road primary school in Luton, in October 1996.
Mrs Davies, whose family home is in Stevenage, Hertfordshire, became the focus of a national debate about school discipline when news of her behaviour was disclosed.
She resigned after being admonished by Mr Russell and warned that she could be sacked if such an incident was repeated.
Mrs Davies, now head of a preparatory school in Kenya, is claiming constructive dismissal against the governors of Tennyson Road school. She said the head's admonishment of her made it impossible for her to carry on working at the school.
Mr Russell had acted as "detective, prosecutor, judge and jury" and disciplined her against the wishes of school governors, she said.
The school's governing body is contesting her claim for constructive dismissal at a hearing in Bedford. The case is expected to last three days.
Mrs Davies said the five-year-old boy, who had special needs, was being repeatedly bullied by other children.
The boy was asked if he wanted to forgive the boys or tap their hands with a ruler after a bullying incident. He opted for the ruler. Parents of five of the boys who were hit then complained and asked for Mrs Davies to be disciplined, the three-man tribunal heard.
The headmaster had made it clear that he was backing the unhappy parents, and Mrs Davies said she felt betrayed.
"I sat in my kitchen staring at the clock, contemplating suicide. I just could not believe that my care for children, other people's children, had ended in this happening to me," she told the tribunal.Reuse content