It has emerged that headteacher Marjorie Evans - convicted then cleared of hitting a pupil - has faced new allegations.
Marjorie Evans' case became a high profile 18-month saga
But the CPS examined claims by an 11-year-old that Mrs Evans pulled her hair at St Mary's School, Monmouthshire, and said there was no case to answer.
In 2000, Mrs Evans was convicted but later cleared of slapping another pupil.
The NUT now says it wants a review of the way allegations are handled and is concerned about "malicious claims".
Secretary of the NUT Cymru Gethin Lewis said: "Increasingly teachers are being seen as easy targets for malicious allegations.
"A way must be found of separating the obviously unfounded allegations from the more serious kind as early as possible.
"The allegation against Mrs Evans was made in September 2003 and it is only now that she has been told that there is no case to answer. The stress which this has caused is unreasonable and unacceptable."
Marjorie Evans ordeal began back in September 1999 when a 10-year-old boy, with attention deficit disorder, accused her of slapping him during a confrontation.
Education chiefs suspended her from the job she had held for the past 13 years.
At court in July 2000, was convicted of assault but claimed he had tried to punch and head butt her and that he was a danger. Two months later Mrs Evans was cleared on appeal and reinstated as headteacher in March 2001.
Talking about the unfounded, new claims against her, she said: "I'm very angry at the moment, I'm just trying to calm down. I was shocked to be accused again.
"The allegation is total rubbish and untrue because nothing ever happened.
"But it seems that teachers are still there to be shot at. Because of the current situation in schools, teachers who need to reprimand a pupil have another adult with them as a witness.
"I am happy to be at the school - the children are wonderful and are usually very well behaved."
The NUT's Gethin Lewis added: "We are seeking an urgent meeting with Chief Executive of Monmouthshire County Council to discuss procedures."
He emphasised that the council had acted properly and the investigation had been handled by senior officers of Gwent police.
At no point has Mrs Evans been suspended.