The Ministry of Education appointed a limited statutory manager in June this year to take control of the 66-pupil school following staff allegations of bullying by the principal.
Mrs Casey went on sabbatical to Auckland University soon after the manager arrived, a move in place before the manager was appointed and in no way related.
An Education Review Office report said there were issues between the board and teachers, and the principal and teachers, that led to "dysfunctional" relationships at the school.
Former board chairman Bruce James said he and four other board members resigned on Sunday because they had come to the realisation the situation was "unfixable".
He said a "toxic environment" had developed within the school which was causing it to become a "sinking ship".
Many parents at the school were now investigating sending their children to another school, he said.
A parent, who wished to remain anonymous, said about a dozen parents were concerned about the return of Mrs Casey to the principal's position in term 1 next year.
The parent said many of the teachers were also planning to leave the school if Mrs Casey returned.
Mr James said he too had heard teachers were planning to resign but, as yet, there had been no official resignations handed to the board.
"I can't say I've enjoyed my time on the board, as from the outset, meetings were fraught with problematic decisions and discussions that I felt were far outside what I originally signed up for.
"It has started to impact on my health and, at this moment, my hands are tied, as the [limited statutory manager] has all jurisdiction over employment matters.
"Once this toxic environment has become a more pleasant one again, I am more than happy to return to the board if needed, and continue the matters of governance, for which I was first elected."
Mr James praised the teachers at Rotary Park, saying they had remained resilient during the turmoil and had continued to educate the school's pupils with professionalism.
Limited statutory manager and Timaru business director Cleave Hay said the situation was disappointing and frustrating.
He said the Ministry of Education had been advised of the board's "decimation", and he believed the ministry would now appoint a commissioner to take over the governance of the school until a new board could be elected.
In the meantime, he would continue to work through the issues affecting the school, which surrounded a formal complaint laid by a staff member about working relationships within the school.
Mrs Casey could not be contacted for comment.
