Their union says it will nonetheless continue bargaining with the government and trustees this summer and hopes to reach an agreement before the new school year.
The Ontario English Catholic Teachers’ Association, which represents some 50,000 teachers, says there has been little progress on key issues that would affect students' learning conditions.
Its members voted 94 per cent for a strike mandate in April.
Several major teachers' unions have listed class sizes, control over teacher preparation time and hiring practices as sticking points in negotiations.
They have raised the possibility of September strikes, which could make for a tense summer of bargaining.
"OECTA members have been without a new contract for one year, and we are discouraged with the slow pace of negotiations," union president James Ryan said in a statement.
"We are optimistic that filing for conciliation will provide the impetus necessary for meaningful negotiations to continue, and that we can avoid disruption to classes in the fall."
More than 70,000 public high school students were out of school for weeks this spring due to strikes in the Toronto and Sudbury areas.
The Ontario Labour Relations Board eventually ruled that the teachers' strikes in the Durham and Peel regions and the Rainbow District were illegal.
In an attempt to avoid further strikes in those boards this school year, the Liberal government enacted back-to-work legislation.
Elementary teachers have been on an administrative strike since May.