An escalating teachers' labour dispute in B.C. is set to become even more confrontational, with the employer announcing a partial lockout beginning next Monday.

That could put extracurricular trips and activities, including high-school graduation ceremonies, at risk and make it impossible for teachers to mark exams or complete report cards, a B.C. Teachers' Federation spokesperson said.

"Effective May 26, 2014, and continuing until further notice, your members will be locked out as described in this letter," Michael Marchbank, public administrator for the B.C. Public School Employers' Association, said in a letter to teachers' federation president Jim Iker.

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The letter, dated May 21 and posted on the employers' association website, says that under the partial lockout, teachers would not be allowed to come to work more than 45 minutes before classes start or stay later than 45 minutes after classes end except for an urgent safety issue. If no agreement is reached, a full secondary school lockout is scheduled for June 25 and 26 followed by a lockout for all schools on June 27.

The two sides are in contract negotiations. Teachers started a job action in April, refusing to perform some administrative work. Rotating strikes are due to begin on Monday.

BCPSEA, the bargaining agent for the province's 60 school boards, also said it would dock teachers' pay by 5 per cent as of Monday over the job action, and by 10 per cent in the event of rotating strikes.

In the letter, Mr. Marchbank says BCPSEA was left with "no option" after the federation did not alter its bargaining position in response to recent changes to employers' offer.

The letter maintains the teachers' federation is seeking total compensation increases of 21.5 per cent over four years and that its demands on class size and composition are "enormously expensive" and would amount to $2-billion a year by the fourth year.

A BCTF spokesman said the union's lawyers are reviewing the letter, but noted a lockout could affect the ability of teachers to mark provincially required exams for Grade 10 and Grade 11 students.

"There are lawful opportunities to go to lockouts and strikes when parties are in a dispute," said Maynard Witvoet, the president of the B.C. Industrial Relations Association, a non-partisan debate group on labour issues. "With the teachers looking at rotating strikes, a lockout would be an option for an employers."

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The employer is required to give at least 72 hours notice.

The potential lockout will push questions about the logistics and legality of the proposed wage rollback to the fore.

"The government hasn't been clear on exactly who would do that [the wage rollback] and how they would do it," Geoff Johnson, a former school superintendent, said on Wednesday.

Individual school boards that implement the wage cut could be a target for legal action, Mr. Johnson said, adding that the teachers' contract would remain in effect even though they were on strike.

"Is it up to the 60 school districts to embark on a course of action that would break that contract, or do something opposing that contract? I think they would be foolish if they did," Mr. Johnson said.

The teachers' federation has said it expects the wage rollback to wind up before the Labour Relations Board.

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School administrators said implementing a province-wide wage reduction would be complicated and fraught with potential for error.

Districts around the province use different payroll systems and teachers are paid at different rates, while some are on leave, working part-time or changed jobs during the school year.

"So it's not quite as simple as if they [the province] were to give everyone a 1.5-per cent salary increase and you just go and program that in," Vancouver School Board chairwoman Patti Bacchus said on Wednesday. "It would require, I think, a bit more detailed approach to get it done accurately. ... There's only a few pay periods left in the whole year, so it's not going to be a simple thing to implement."

As of Wednesday, Ms. Bacchus had not received any instruction from the province or the BCPSEA about implementing a wage rollback.

The contract for B.C.'s 41,000 public school teachers expired last June.

Last Thursday, the province backed away from its previously-stated goal of a 10-year deal and offered teachers a $1,200 signing bonus if a deal was struck before the end of June. A day later, the chief negotiator for the province said B.C. was prepared to dock teachers' pay by 5 per cent in retaliation for the job action. The BCTF then said it would begin rotating strikes.

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"It would have been nice if they [the province] had left the proposal from Thursday on the board for consideration and not immediately moved to a threat or coercive kind of proposal," said Daniel Laitsch, an associate professor in Simon Fraser University's faculty of education.

The situation is coloured by a long-running court fight that has resulted in the province and the BCTF approaching the bargaining table from sharply different perspectives, Dr. Laitsch said.

The court case relates to contract provisions that allowed teachers to negotiate class size and composition.

In January, a Supreme Court of B.C. judge reaffirmed a 2011 court ruling that found provincial education legislation introduced in 2002 that removed those bargaining was unconstitutional. The province is appealing the decision.

The proposed wage rollback is designed to put pressure on the teachers' union to reach an agreement, said Teresa Rezansoff, president of the B.C. School Trustees Association, the umbrella group for B.C.'s 60 school boards.

"It's unfortunate, but it is a necessary step that is being taken by BCPSEA to try to bring pressure to the table and hopefully that will result in a resolution at the bargaining table," Ms. Rezansoff said. "We don't want to go into September without having certainty in our system, that's for sure."

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With a report from from Justin Giovannetti