SHIRLEY -- When selectmen and Finance Committee members talked about budget matters Monday night, the brief joint session centered on the Ayer-Shirley Regional School District and a looming half million-dollar bill on the Shirley side.

With Finance Committee Chairman Frank Kolarik and member Mike Swanton at the selectmen's table, Superintendent Carl Mock and School Committee Chairwoman Joyce Reischutz sketched a problematic picture that could get worse if the town can't meet its obligation.

Acting Chairman David Swain said the school is a "major concern" for the current and next fiscal years' budgets.

"We knew our contribution would step up gradually, he said, as the number of Shirley students rises in relation to Ayer's share of the student population via a formula spelled out in the regional agreement. Both member towns voted to amend that formula last year.

Now comes a spike in Shirley's state-mandated "required local contribution," which jumped to 100 percent this year. With the formation of a region school district, it's based on student numbers that now include almost all school-age children eligible to attend district schools, even if they don't. Exceptions include students attending Nashoba Tech, the regional vocational high school district that Shirley and Ayer both belong to.

Besides the RLC uptick, state education aid -- Chapter 70 funding -- fell this year.

The Department of Education says Shirley was "significantly below minimum net school spending," Swain acknowledged. But apparently the sudden hike is a surprise. "We've been working gradually toward" closing the gap, he said. "But this year, 100 percent is required."

The town's contribution to the regional school district now comes to about $500,000," he said. "That's a tough nut ... to swallow at present."

Among the factors figured into the contribution formula is the school budget itself. But Mock said that "as a raw number," the school budget is "not going up that much."

It is higher, however, due to an unexpected hike in special-education costs, up by about $550,000 this year. That amount was "not previously budgeted for" and includes out of district SPED placements for current and new students, Mock said.

Asked if the school budget was "level funded," Mock said not exactly. It includes added staff hours for a SPED program that provides behavioral therapy to students who might otherwise need out of district placement, at a far greater cost and needed supplies that were cut last year. Mock called these items "extremely important."

The budget request also includes funding for an indoor track program serving about 30 kids that now runs on volunteer power and fundraising.

But there are about a dozen fewer positions now than in fiscal 2012, Mock said.

"We're asking for a 3.6 percent increase with practically no more money from the state," Mock said. Ayer's share will go up by three percent, while Shirley's is 11 percent higher. "That's the bad news," Mock said.

On the good news side, Shirley's required local contribution to Nashoba Tech, with fewer students attending this year, went down by $13,000. However, the state formula for vocational/technical education is different than it is for other public school districts and is based on a lower per-pupil cost.

Citing facts and figures from a handout presented to the Finance Committee last week, Mock also pointed out the district stabilization fund intended to cover Shirley's portion of debt costs for the ongoing, voter-approved high school renovation and addition project.

Now at the architectural design stage, work is expected to start soon, with completion in a couple of years. The building project's $56 million price tag is split between the two towns per a formula in the regional agreement that holds Shirley harmless in the first year of the debt, then ups its share over time. The fund Mock referred to is part of that plan.

"What's the principal driver" of the increased local contribution cost? Kolarik asked.

Mock said that, overall, the state is seeking 59 percent of foundation budget.

In terms of a community's RLC, average income as well as growth factors are figured into the state formula, Reischutz added. 

Principal Assessor and Finance Committee member Becky Boucher asked why the town can't count debt for the middle school -- built by the town about seven years ago and still on the books -- as part of its required local contribution.

As Mock explained it, the state formula is based on the school's foundation budget and the community's expected effort to support it. Building projects would be "above and beyond," in that sense, he said.

"The state is concerned with ... student education," Reischutz added. Basically, it's about money spent on kids' education, not on buildings or transportation, she said.

If Shirley is below RLC, Boucher asked why it's necessary to "work up" to meet Ayer's. Because it's in the agreement, she was told. In that case, she suggested bar graphs to explain the scenario at Town Meeting so that voters can see how it works.

Reischutz noted that Shirley isn't the only Massachusetts town facing this problem.

Swain agreed. But by the same token, it can't be totally solved at the local level, he said. "We need to contact our state representatives."

The state had promised Shirley "more money" that was "taken away" this year, he said, including $90,000 in annual MCI mitigation funding. "Now, this hurts even worse and they need to know that," he said.

Mock agreed that pitches to DOE and state representatives are in order.

Given the "confluence of events," Swanton said he didn't see any way the town could come up with $500,000 for the school this year. He also said SPED costs were too hefty for such a small community. "I think we need to remind our legislators again," he said.

With Shirley's share suddenly ratcheted up so substantially, Swanton also said the two towns may need to "adjust the regional agreement" so the district can move forward.

The other town may be thinking along those lines, too, Administrative Assistant Kathi Rocco said, noting that Ayer Town Administrator Robert Pontbriand recently spoke to her about it, stating that the leadership group needs to meet to talk about the situation.

"I think the issue is serious enough not to overlook anything," Mock said.

Concluding the discussion, Swain said he planned to draft a letter, with the Finance Committee, to send to state representatives. Sen. James Eldridge and State Reps Jen Benson and Sheila Harrington were invited to attend the upcoming School Committee meeting.