BOSTON (AP) — Of all the bills that stalled out during the Massachusetts Legislature's just-completed formal session, perhaps the one creating the most heartburn is the failure to reach a deal on overhauling the state's 25-year-old education funding system.

Education advocates, parents and teachers unions had hoped to revamp the funding formula — known as the "foundation budget" — a key element of the landmark 1993 Massachusetts education reform law meant to smooth out educational disparities between wealthier communities and poorer ones.

To address the issue, the state set up a Foundation Budget Review Commission, which found the original formula underestimated education costs by up to $2 billion every year. The commission made a series of recommendations to overhaul and update the foundation budget in a report issued three years ago.

Even with all that preparation and discussion, the Massachusetts House and Senate — each of which had passed their own version of an overhaul — failed to reach a final compromise deal in the waning days of the 19-month formal session.

The Senate bill would have increased annual spending on education by about $1 billion when fully implemented. The House bill would spend about $500 million over five years to help school districts better cover the costs of special education and employee health care.

The criticism was swift — including talk of possible lawsuits by districts seeking more funding from the state.

"This week the Legislature failed to perform its most essential duty: to provide sufficient funds, as demanded by our Constitution, to 'cherish' our public schools," Massachusetts Teachers Association President Merrie Najimy said in a statement. "Our students, especially those who are most vulnerable, will go another year without getting what they need and deserve."

Democratic Sen. Sonia Chang-Díaz, who served as the Senate chair of the Foundation Budget Review Commission, said lawmakers had been working on the issue for eight years and the Senate did its best to reach a final deal, offering what she said were "multiple versions of major concessions."

"Yet, in the end House leadership rejected all our offers, moved the goal posts, and then killed the bill completely," Chang-Diaz said in a statement early Wednesday morning. "I've seen a lot in my 10 years in this building, but I've never seen so many rationalizations and double-standards employed to avoid doing what's right for kids."

The three House negotiators — Democrats Alice Peisch and Claire Cronin and Republican Kimberly Ferguson — said the House was also "disappointed we were not able to come to workable resolution" but said the House remains committed to implementing the Foundation Budget Review Commission's recommendations.

The House team said efforts to come up with a final deal were "complicated by new information obtained from the Department of Elementary and Secondary Education and the exceptionally complex nature of recalculating various increments to the formula as we traded proposals."

The three said the House is also dedicated to ensuring additional funds are included in the state budget that begins next July for English language learners and low-income students.

That refers to another of the key differences between the House and Senate bills.

The House bill would have increased spending to help districts better cover the costs of special education and employee health care.

What the House bill didn't do is set aside funding for two other key elements of the foundation budget report: increasing funding for schools teaching English language learners and schools with low-income students — instead calling for the hiring of a research consultant to figure out how much should be spent on those two groups.

The Senate version addressed all four elements — including the English learners and low-income students. Chang-Diaz and others argued that the issue had been studied enough already by the foundation budget commission.

David Jones, a history teacher and member of the teacher advocacy group Educators for Excellence-Boston, chided lawmakers for inaction.

"Policymakers have failed to address inequities in funding that harm students in Boston and other urban centers across the commonwealth every day," Jones said in a statement. "We need a foundation budget with the basic changes necessary to move the needle in our classrooms — instead, we received inaction."

The Legislature won't meet again in formal session until January.