Some full-day kindergarten classes in Ontario are crammed with as many as 40 students, so many that the children can't fit on their classroom carpet for group time, raising questions about the Liberal government's ambitious program.

As Premier Kathleen Wynne's government rolls out full-day kindergarten to all schools this fall, documents obtained by The Globe and Mail through freedom-of-information legislation show the Ministry of Education has been inundated with complaints from parents and educators about large classes impeding learning.

Ontario has staked its reputation on the success of the program, even though critics have characterized it as an expensive form of government-subsidized daycare that the Liberals, facing a $12.5-billion deficit, can ill afford. Alberta and Manitoba have decided against full-day kindergarten, basing their decisions on the cost, while Newfoundland and Labrador recently pledged to implement full-day senior kindergarten in 2016. The hope is that the investment will pay off with higher graduation rates and improved academic outcomes.

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But documents obtained by The Globe show that Ontario's early learning program is grappling with huge issues. About 640 kindergarten classrooms, or 8 per cent of those that introduced the program, had more than 30 children in the past academic year, according to a confidential briefing note to the minister in January. One senior kindergarten classroom at the Ottawa-Carleton District School Board had 40 children – the biggest class for that age group – according to the ministry documents. The board told The Globe that it ended up dividing the class.

In letters to ministry officials, parents described classrooms as "understaffed daycares," "chaotic," "overcrowded" and "hostile" environments for learning. One parent opted to put a child in a daycare kindergarten program, which has a lower student-teacher ratio. Another parent kept her four-year-old daughter out of school.

"I strongly believe in the public system," the parent wrote, "however the government has really let down these kids in jk/sk as well as their teachers."

In Ontario, daycares are required to have one educator for every eight preschoolers, and primary classes have a cap of 23 students. There is no cap for full-day kindergarten classrooms. Instead, the government stated in its response to various complaints that school boards are required to maintain an average class size of 26 across the board. Full-day kindergarten classes have a teacher and an early childhood education worker.

Research shows that small primary classes have little impact on achievement; good teachers matter more. The Liberals, however, have pointed to smaller class sizes as a key education platform, saying children get more attention and do better in school. The government excludes full-day kindergarten from those calculations.

Martha Hradowy, who represents early childhood education workers at Windsor's Greater Essex County District School Board, said the situation has become so dire that some schools have created learning areas for full-day kindergarten students, which hold about 100 children in one large room. Classrooms are divided into four separate corners.

"The high levels of noise do impact learning, and speech development," Ms. Hradowy said. "We believe that the ministry needs to step in."

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A spokeswoman for Education Minister Liz Sandals said the government has no plans for a hard cap, despite demands from parents and educators.

Ontario's full-day learning program incorporates two years of a revamped curriculum for junior and senior kindergarten. The program has been introduced in phases over five years, and the government is spending more than $1.45-billion in capital costs to expand and retrofit schools, on top of millions in operating dollars.

The government has defended the program by pointing to a study that it funded showing children enrolled in the first two years of the province's all-day learning program were better prepared for Grade 1 and have stronger language development and better communication and social skills. But other studies suggest children in a full-day program are academically no better off in the primary grades than those who attend a half-day program.

John Paterson's five-year-old son, Charlie, will be attending full-day senior kindergarten at Toronto's Swansea Public School this year. Not only will Charlie's class have 30 students, but he will be in a large room that will hold two classes, or 60 kindergartners. Mr. Paterson said he and other parents have expressed concern to school officials, and would prefer the government cap the class size.

"I think the benefit of a large classroom is my son gets a lot of fun social interaction," Mr. Paterson said. "But I suspect, and a reasonable person would surmise and conclude, that the actual learning and communication of knowledge to each student is diminished with 30 kids per teacher [and an early childhood educator]."