The 44-year-old mother-of-four (pictured right) set a legal precedent after winning a battle to return to Langtree primary in Torrington, north Devon, on reduced hours.
An industrial tribunal found that Devon County Council and governors at the school had sexually discriminated against Mrs Marshall and unlawfully refused to allow her to pursue her career.
The tribunal declared that the condition of a five-day working week was discriminatory. It was the first case of unlawful discrimination involving a female head not being able to return to work on less than a five- day week.
But her union, the National Association of Head Teachers, Devon County Council and the school's governors have been unable to agree a deal for her to return on reduced hours.
Mrs Marshall has been paid for three days a week pro rata since February when her maternity leave ended although she has not been in school for 18 months.
She will be seeking compensation, in excess of pound;100,000, at a remedies tribunal on February 9, 2001. Mrs Marshall will not return to work on the terms offered by the local authority and governing body.
Ian Poole, NAHT soliitor, said: "It is very disappointing. We are disappointed, and she is disappointed although we clearly established the principle that headteachers can job share."
The sticking point in talks, which included a lengthy meeting between the union and the council, was the local authority's insistence that the job-share arrangement was temporary. Devon wanted Mrs Marshall to return to full-time work by April 23.
That date coincides with the end of the Easter holidays. If she had agreed to it, she would have been on a three-day week for a term and a bit.
Mrs Marshall has two teenage children and two children under six. The youngest is one year old.
Mr Poole said the NAHT would be seeking around pound;30,000 per annum for her until retirement less tax and alternative earnings that she might have earned.
"It is difficult to envisage her returning to headship. We will be seeking compensation in excess of pound;100,000," he said.
"It is disappointing that the governing body and local authority are dead set against this."
A Devon spokesman said: "We and the governing body have offered Mrs Marshall's advisers a fair and reasonable proposal that would enable her to return to school now and meet her childcare needs. It is regrettable that the resolution of her case will have to a wait untila further hearing.."