Unionized elementary school teachers in Ontario say this week's heat wave has made conditions in many classrooms "unbearable" and it's taking a toll on students who are stuck in schools without air conditioning.

The Elementary Teachers' Federation of Ontario says the extreme temperatures affect children's ability to learn and can cause health issues.

The union is calling on the province to take steps to relieve the heat, which could include installing air conditioning systems or setting up cooling stations.

It says all schools should have a heat stress plan and a maximum indoor temperature limit after which they must take action, including shutting down for the day if necessary.

Much of southern Ontario is under a heat warning, with Environment Canada calling for temperatures above 30 C. Cooler weather is expected later this week.

"While we recognize that air conditioning can be expensive, there are significant costs in not fixing the problem. The current haphazard approach compromises teaching and learning conditions and can have serious health impacts for children and staff including asthma," said Sam Hammond, the union's president.

When asked about the issue, Premier Kathleen Wynne said she believes teachers will take action if the heat presents a health issue for any students.

The government has spent billions of dollars on school upgrades and it's up to individual boards to decide how to spend that money, she said.

"It's very hot right now, I recognize that," Wynne said. "There has been money that has flowed to school boards where decisions are made about changes in their buildings."

The Toronto District School Board says it has cancelled scheduled outdoor athletic events for the day due to the weather.

Of the board's 584 schools, approximately 125 are fully air conditioned, some are partially air conditioned, while others have no air conditioning.

The board says it has been taking a number of steps to manage the heat and keep students as cool as possible, including using fans where possible, turning off lights and computers where possible and rotating students into air conditioned areas such as libraries, if possible.

Meanwhile, the Ottawa-Carleton District School Board, which also has some schools that don't have air conditioning, said it has been in touch with the city's public health department regarding strategies to manage the heat. Those strategies including ensuring staff are aware of the symptoms of heat exhaustion, dehydration and heat stroke.

The board also said it will ensure plenty of opportunities for students to drink water throughout the day.