No evidence to show conviction damaged the profession, says the NAHT.
The union representing head Paul Davies, who was struck off this week for a dangerous driving conviction two years ago, is to investigate the landmark ruling by the General Teaching Council for Wales.
The head of Cwmdare Primary in Rhondda Cynon Taf was banned from his post on Monday after a disciplinary committee hearing of the regulatory body.
He had been allowed back to work last month while on licence from prison after serving three months for driving "like a lunatic", an act that left one of his victims in a wheelchair.
But Anne Hovey, National Association of Head Teachers Cymru regional officer, said there was no evidence to support the verdict that the teaching profession had been damaged. "There's no such perception in the local community," she said. "I am confused as to how they came to that conclusion."
Ms Hovey said she needed to examine the wider implications of the judgment, particularly in the grey area between teachers' private lives and public duty. She said decisions made across the UK in professional conduct cases such as Mr Davies's seemed to vary in severity.
Mr Davies was described as a boy racer by the judge, who jailed him after he collided with three cars in torrential rain. Victim Kelvin Palmer was left unable to walk. Mr Davies was sentenced to 15 months in prison in May 2006 at Merthyr Crown Court.
Disciplinary committee chairman Jacquie Turnbull said the ban was necessary to maintain public confidence in the teaching profession. Mr Davies is currently considering an appeal against the decision.
Meanwhile Mr Palmer, in letters to the GTCW, has claimed Mr Davies did not inform his governors or RCT local authority about the charges against him. His complaint has been taken up with the office of the children's commissioner for Wales.
After the hearing, Mr Palmer's wife Anne, a teacher at Treorchy comprehensive, said: "I think Mr Davies's conviction - and that he was allowed to return to his job - sent out the wrong signal."
As TES Cymru went to press, a spokesperson for RCT said a meeting would be held to discuss the GTCW decision and it would be up to the governing body to find a replacement head in the meantime.
Leader, page 28.