The decision is the latest court victory for the union, but in this case the issues focused on two types of special employment benefits covering pregnancy and parental leave.
The union argued the B.C. Public School Employers' Association was discriminating against female teachers in the Surrey school district, which is located outside of Vancouver, by denying them parental benefits if they had already received pregnancy benefits.
The employer asked the top court to side with a lower court ruling, which ruled there was no discrimination, and dismiss the teachers' appeal.
Not long after oral arguments concluded Wednesday morning, the justices announced they would allow the teachers' appeal. A subsequent Supreme Court of Canada news release announced the oral judgment would follow within 48 hours.
Kasari Govender, executive director of West Coast LEAF, a women's advocacy group that intervened and argued the employer's actions were discriminatory, said the manner of the ruling was highly unusual.
"In fact, I've never heard of it," she said. "It has happened from what I understand now, but it shocked us all that they delivered the judgment from the bench.
"Normally, it would take months for them to write a decision."
The employers' association said in an email that it is not yet prepared to comment and is reviewing the ruling.
Union president Jim Iker said the ruling was a victory for all working women who are pregnant or may become pregnant in the future, not just teachers.
"The Supreme Court of Canada said employers cannot discriminate against pregnant women and that benefit plans for new parents must be consistent with the Charter of Rights and Freedoms," he said in a news release.
The case started as a grievance in February 2011 between the Surrey Teachers' Association, which is represented by the BCTF, and the Surrey school board, which is represented at the bargaining table by BCPSEA.
The teachers argued the school board was discriminating against mothers because it didn't provide 15 weeks of parental benefits, topped up to 70 per cent of employees' salaries, if they had already received pregnancy benefits.
An arbitrator upheld the union's grievance in April 2011, but the employer appealed and the case ended up in the province's top court.
The B.C. Court of Appeal ruled in September 2013 that the arbitrator erred and dismissed the teachers' grievance, and the case ended up in the Supreme Court.
Govender said the decision has restored the arbitrator's ruling, which stated the benefits scheme was discriminatory and breached the province's human rights code and the charter.
She said the parties now need to return to the bargaining table to discuss how they can make the benefits scheme compliant.
"If they can't resolve it at bargaining then they have to bring it back to an arbitrator," she said.
The B.C. Teachers' Federation is coming off a series of victories in the province's Supreme Court. In 2011 and again earlier this year, Justice Susan Griffin ruled the province acted unconstitutionally when it deleted hundreds of clauses over working conditions.
Both sides resumed their battle in the B.C. Appeal Court last month and regardless of the decision the case is expected to head to the Supreme Court of Canada.