A row over management techniques at a Roman Catholic high school in Liverpool resulted in a one-day strike this week, with the threat of more to come.
A senior official at the NASUWT branded the school's long-serving head, Sister Ann Marie Gammack, "unChristian" and "arrogant".
St Julie's Catholic High School in the Woolton area of the city was shut on Tuesday after teachers voted to stage a 24-hour walkout in protest at the way Sister Ann Marie runs the 1,300-pupil state secondary.
Local union officials say teachers were angered when Sister Ann Marie, head of St Julie's for 18 years, introduced "excessive classroom observations" at the start of term. These are believed to have been introduced in the maths department after the headteacher said grades in in last summer's GCSEs had failed to come up to scratch.
A school spokesman described the "excessive observation" as a way of ensuring that pupils get the "best possible education".
A quick settlement to the dispute seems unlikely after Jerry Bartlett, deputy general secretary of the NASUWT, accused Sister Ann Marie of being the "worst manager" he had encountered in more than 40 years as a union member.
He said: "We have never dealt with a school where the conduct of the management has been so unacceptable. This arrogant headteacher has never answered our letters."
Mr Bartlett accused Sister Ann Marie of denying teachers promotion, bullying them and reducing them to tears. He also said she had brought the Roman Catholic education service into disrepute. "She wrote to one of our members calling him 'offensive' and 'un-Catholic'," said Mr Bartlett. "I am responding in kind by calling her 'unChristian'."
Mr Bartlett said that his members had nearly gone into official dispute on five previous occasions. "They have always previously backed down," he added, "but now they've had enough. It is a wonderful school with very committed staff, but enough is enough."
This week Mr Bartlett wrote to senior figures in the Roman Catholic Church, including Patrick Kelly, the Archbishop of Liverpool, Oona Stannard, the director of the Catholic Education Service, and the Sisters of Notre Dame - founded by St Julie, to whom the school is dedicated.
Mr Bartlett said further industrial action could follow if the dispute was not resolved soon, but the union hopes to meet Sister Ann Marie in the next few days to start talks on resolving the dispute before the term ends on December 19.
Any resolution, Mr Bartlett said, would include the provision that "we intend to monitor her behaviour for the next two years".
The St Julie's spokesman said: "The school is committed to doing all it can to make sure pupils get the best possible education and has recently introduced additional measures to monitor the standard of teaching in lessons.
"We are in discussions with the NASUWT about the issue they have raised. It is regrettable the union has chosen to disrupt pupils' education by taking industrial action rather than sitting down to resolve the issues."
A 2007 Ofsted report said of St Julie's: "The headteacher, supported by a growing leadership team, both in numbers and skills, provides strong and incisive leadership. She has a clear vision for the school and sets very high expectations for staff and students. The school's self evaluation clearly identifies strengths and weaknesses and, in pursuit of raising standards is occasionally, though understandably, hard when judging itself."