The teacher, 48, who cannot be named for legal reasons, was convicted at Hamilton Sheriff Court, near Glasgow, after admitting hitting his daughter.
Sheriff Dan Russell told the man he had caused the child "unnecessary suffering" when he smacked her in the waiting room of a health centre on Christmas Eve 1998.
He said that striking the girl on the bare bottom, combined with the severe force of blows, went beyond the corporal punishment a parent was entitled to use and constituted an assault. Evil intent had been proven.
"The blows were clearly sore and must have caused her pain. In my opinion this was unnecessary suffering. I therefore find the accused guilty of the charge," the sheriff said. He deferred sentencing until after social reports.
A children's charity, Children First, immediately called for greater efforts to be made to make parents aware of the rights and wrongs of disciplining children. "We have been arguing for a while that the law should be tightened up to make it clear what does constitute an offence and what does not," said a spokeswoman.
The court heard how the police were called by a member of health centre staff, who witnessed the teacher assaulting his daughter when the child refused to have a tooth removed.
Yvonne McEwan, 31, a speech therapist, told the court that she saw the man put the girl over his knee, pull down her pants and smack her several times. She said she heard the child screaming, "Daddy, please don't hit me."
Elizabeth Nisbet, 42, a dental practice nurse, described trying to separate the girl from her father. She said: "The man was getting angrier. I had to move from side to side to keep the girl from him."
After the incident, the man was banned from seeing his daughter over Christmas and told to stay away from the family home for two weeks. He is currently suspended on full pay from his job at a primary school. His future is to be decided by North Lanarkshire General Teachers' Council.
Outside the court, the man said: "Many parents and teachers will be looking over their shoulders for fear of litigation. My greatest consolation is my wife and my lovely children. We are very happy. I regret what I did. However, I don't regret my daughter being relieved of toothache over Christmas.
"I am a conscientious father. I think I am a very good father. My daughter is a courageous and brave lassie, a beautiful child and I also have the best wife. This has had a massive effect on my family."