National Union of Teachers solicitor Graham Clayton said it was outrageous that the head had yet to see the allegations of pupil mistreatment which could spell the end of her 34-year career.
The 10 claims, made after the head was cleared on appeal of slapping a 10-year-old pupil at St Mary's junior school, Caldicot, range from shouting to physical assault.
The NUT could ask for a judicial review on the grounds that Monmouthshire County Council and the governing body have acted unreasonably.
The threat was made after the council failed to met a deadline set by NUT lawyers who had requested details of the allegations. Mr Clayton said:
"After 15 months of this it is time for the LEA and governors to put up or shut up."
Gethin Lewis, NUT Wales representative, hinted that Mrs Evans could turn up at the school on the last day of term, today.
He said: "Marjorie wanted to return to work before the end of term. She is on compassionate leave, and as far as I am concerned she is entitled to return to her post. She has done nothing wrong and is ready and waiting."
Murray Andrews, head of legal services at Monmouthshire, said details of the allegations would be given to Mrs Evans seven days before the disciplinary hearing.