The Ontario Secondary School Teachers' Federation is threatening to withdraw services at seven school boards: Peel, Durham, Halton, Ottawa-Carleton, Waterloo, Rainbow in Sudbury and Lakehead in Thunder Bay.
There is a "high level of frustration" at the table in Durham, so the union has singled it out as the first board that could see a strike as early as April 20.
"We really have announced that we're serious about this," said union president Paul Elliott. "We believe you should be serious about this also, and we've got until April 20 to work toward settling that local deal."
Education Minister Liz Sandals said unions setting strike dates is often just part of the bargaining process.
Premier Kathleen Wynne urged teachers to stay at the table.
"Negotiations are always tough and they're particularly tough in an environment where there are constrained finances," Wynne said. "We knew the bargaining was going to be tough, but we're committed to staying, being part of a strong and honest collective bargaining process."
This is the first round of negotiations with the Ontario Secondary School Teachers' Federation since the province brought in a new bargaining system, with both local and provincial talks.
Sandals said all is well at the central table, where monetary issues are being negotiated.
"I believe we're making progress and it's certainly my intention to stay at the table and to get a negotiated agreement," she said. "I'm hopeful that they'll reach some sort of agreement at the local level as well."
The negotiations are also taking place as Ontario tries to eliminate a $10.9-billion deficit by the Liberal government's self-imposed deadline of 2017-18. To that end, the province has told teachers, public servants and other major unions with whom it is negotiating that any compensation increases must be offset to achieve "net zero."
Elliott said the effective wage freeze is not the main issue, that teachers want improvements to working conditions and are looking for more face-to-face time with students.
The seven boards named by the union have "highlighted their lack of movement and their frustration at the local level," Elliott said. Progress can still be made before April 20, he said.
"I'm optimistic that once you have a deadline and people take it seriously you can start working towards it," Elliott said.
Note to readers: This is a corrected story. An earlier version carried incorrect information on the school boards which could be in a legal strike position later this month.