School committees, superintendents and education advocates statewide are hoping to give families greater access to full-day kindergarten classes for their 5-year-olds, but lean budgets and crammed classrooms are making the expansion difficult.

Most kindergartners have access to full-day classes, officials say. But in many districts, families are charged more than $2,000 for the all-day experience, making it less accessible for children who need the attention the most: those from low-income families.

"Increasingly, school districts are putting them in, but they’re charging tuition," said Margaret Blood, director of the group Strategies for Children, which is sponsoring legislation on Beacon Hill asking for more funding. "The children that often most need it, those families can’t afford to pay it."

The group is advocating for universal access to full-day kindergarten and asking for the state to pay for it.

"I think there are some families who prefer to have their children in a half-day program," Blood said. "That’s why we think it should continue to be a family choice."

The state requires districts to provide the equivalent of a half-day program free of charge, and all do. But what is offered beyond that varies widely from district to district.

Natick is the only town in MetroWest to provide free full-day kindergarten to any family that wants it. Hopkinton and Hudson, on the other hand, offer only half-day classes.

Other districts offer a full-day option, but charge more than $2,000 to pay for teachers and supplies and give tuition breaks to low-income families. Most of those districts have waiting lists for full-day kindergarten because they don’t have the space to accommodate everyone.

"The first issue is we do not have space," Framingham Superintendent Christopher Martes said. The district three years ago closed the Juniper Hill School to trim $1.8 million from the budget and would need to reopen that building to add classrooms.

has a lottery system for families interested in full-day kindergarten. It charges $2,500 per student to upgrade from the free half-day program. That charge, in addition to state grant money, pays for the full-day classes, Martes said.

"If it’s still tuition-based, we could offer more if we had more space, because it wouldn’t be in the operating budget," he said. "If you decided you wanted it to be free for everyone, then you would have to absorb each of the teacher salaries."

Martes said the district would need greater state support for the program to expand.

School Committee member Pam Richardson has said the department should try to reopen Juniper Hill in phases to free up classroom space.

The Marlborough school district, which charges $2,500 for its full-day option, is also stymied by money and space issues. The district would give up its lottery system and offer full-day to everyone "as soon as we have more available resources and more space to be able to expand," Assistant Superintendent Jim Jolicoeur said. "That’s really our ultimate goal."

According to Blood, studies show that kindergartners benefit more from a full-day program. Some families would rather ease their children into a school environment, she said, but her group conducted a poll that showed 81 percent of families believe kindergarten should be full-day.

"There are some parents and some educators who believe for some children half-day kindergarten is a good transition from home to school," Holliston Superintendent Bradford Jackson said. Holliston charges tuition for full-day kindergarten, but has managed to find space for all who want it.

"From an educational point of view, I think it certainly would benefit students to be in an instructional environment longer," he said. "The question is not whether school is important, but how it should begin and when it should begin."