The union representing Toronto elementary school teachers has given the rollout of full-day kindergarten in Ontario a failing grade.

Introduced in 2010, the full-day kindergarten program has been implemented gradually in schools across the province. The goal was to make the program available to 265,000 four- and five-year-olds in around 3,600 schools at the start of the 2014-15 school year.

However, John Smith, the president of the Elementary Teachers of Toronto, says the union has been "inundated" with calls from teachers complaining about classroom conditions.

The issue isn't the program, which Smith says teachers support, but he's concerned that because of classrooms sizes they can't focus on teaching the curriculum, and have been handcuffed into "crowd control" roles.

"Instead of teaching literacy and numeracy, you are dealing with issues with students," he told CTV Toronto.

Last year, eight per cent of full-day kindergarten classes had 30 or more students, and the average class size was about 24 students. Class size numbers for this year have yet to be released.

Smith says large class sizes have led to safety concerns among teachers, and the number of number of teachers who have taken stress-related sick leave has tripled.

"Teachers are being attacked, grabbed, bitten," he said. "Kids fighting with each other, kids running and leaving the classroom."

In the coming weeks, the union will be launching a campaign to reach out to the provincial government and parents. They say a cap on the number students in kindergarten classrooms could go a long way in improving conditions.

The province's minister of education, Liz Sandals, has said no to a cap on classroom sizes in the past, but she is open to the union's concerns.

"Whenever we have a new program, we always look at implementation and feedback," she said. "So, absolutely we'll look at that feedback."

Smith says teachers think kids can benefit from full-day kindergarten, but he says when the average ratio is 13 students to one adult, classrooms are simply too crowded.

"Kids three and four years old need a lot of attention … and large classes don't allow that."