SALEM - The tug-of-war over full-day kindergarten returns to the state Capitol this week, with a key Senate committee on Tuesday hearing a bill to rejigger the state funding formula to reimburse schools for offering all-day programs for their youngest students.

The proposal has a high-profile champion in state Schools Superintendent Susan Castillo, who said she sees it as “a legacy for our state and our children.”

But a powerful group of education advocates is against the proposal, and some researchers say there's not conclusive evidence that full-day kindergarten gives the most bang for taxpayer buck.

Only about 20 percent of Oregon elementary schools offer full-day kindergarten programs, compared to about 65 percent of elementary schools nationally, according to statistics from the U.S. Census Bureau.

And unlike at least 16 other states, Oregon only reimburses schools for the cost of providing a half-day program; schools that have opted to expand kindergarten usually cobble together funding from federal sources, grants and tuition charged to parents.

Castillo argues that giving schools the money to offer all-day kindergarten as an option will pay off over time by helping the neediest students, especially those learning to speak English, those with learning disabilities and those from poorer homes.

Shellie Currier, a first-grade teacher at Ochoco Elementary in Prineville, said she's seen noticeable improvement in her students since the school began full-day kindergarten five years ago. All-day classes are optional, but only a handful of families opt out, she said.

“It has really made such a huge difference for us; I can't imagine going back,” Currier said. “They soak up so much information. We have kids coming to first grade who are reading, so we can concentrate on their beginning math skills, and getting them to be a great reader.”

But full-day kindergarten doesn't come cheap. Castillo estimates that fully phasing in all-day kindergarten over the next two school years would cost about $50 million, to pay for teaching time and adding classrooms.

And education advocates like the Oregon Education Association, the state's largest teachers' union, and the Confederation of Oregon School Administrators - who are normally in lockstep with Castillo on policy issues - plan to testify against her proposal.

Laurie Wimmer Whelan, a lobbyist for the OEA, said the concern is that making changes to the state funding formula for schools - which weighs different factors to determine how much per-pupil money schools should receive - could lead to an unbalanced system, unfairly penalizing schools that don't move to full-day kindergarten models.

“If you tamper with the formula to incentivize particular programs, then you move to a system of rewards and punishment,” she said.

And Chuck Bennett, a lobbyist with the school administrators' group, said there were also bricks-and-mortar issues to consider: fast-growing districts like those in Portland suburbs have to offer kindergarten in shifts, one in the morning and another in the afternoon. Moving to a full-day system would require classroom space some districts just don't have, Bennett said.

Both Bennett and Whelan said they like the idea of all-day kindergarten, but suggest that districts should be able to draw on a special school improvement fund to pay for it. Such a fund would also let districts choose other programs to invest in instead, like vocational education or mentoring new teachers.

Full-day kindergarten is one of a slate of proposals on early childhood education likely to surface before legislators this session. Nonprofit education groups are pushing plans to lower class sizes in the earliest grades, while Gov. Ted Kulongoski has proposed expanding the state's Head Start program, funded by a rise in the minimum taxes paid by corporations.