OLYMPIA — Lawmakers have said they can figure out how to adequately fund public schools during their 2015 session, and the state Supreme Court decided Thursday to let them prove it.

In an unprecedented decision, a unanimous high court found the Legislature in contempt for violating an order to produce a detailed plan for funding Washington schools.

But justices didn’t punish lawmakers. They agreed to wait and see what happens next session and, if the Legislature fails to deliver, impose sanctions next year.

“In the interest of comity and continuing dialogue between the branches of government, the court accepts the state’s assurances that it will be compliant by the end of the 2015 session,” Chief Justice Barbara Madsen wrote in the five-page order issued Thursday morning.

“If the contempt is not purged by adjournment of the 2015 Legislature, the court will reconvene and impose sanctions or other remedial measures,” she wrote.

The ruling is the latest development in a lengthy legal fight to force the state to meet a constitutional duty to amply fund a program of basic education for Washington public school students.

It comes a week after justices conducted a hearing about whether to hold lawmakers in contempt and sanction them. Potential punishments they discussed ranged from fines to invalidating tax breaks to free up money for schools.

“This unprecedented action by the Supreme Court is a critical moment in our history,” Gov. Jay Inslee said in a statement on Thursday. “No one should be surprised, yet no one should minimize the court’s order.”

Thursday’s decision might prove to be the one that compels decision-making by a Legislature hamstrung by political and philosophical differences.

“I am so pleased the Supreme Court is going to hold our feet to the fire to keep our promise,” said Sen. Rosemary McAuliffe, D-Bothell, who serves on the Senate education committee. “I would have been disappointed if the court did nothing.”

Thomas Ahearne, attorney for the McCleary family and a coalition of education organizations whose suit ignited the battle, praised the court’s ruling.

It leaves no doubt the court will act, and at the same time it cleverly shifts the onus for compliance into the lap of lawmakers, who have insisted that next year, not this year, is when they will be most able to get a grand agreement on funding, he said.

“The state’s excuse for why they shouldn’t be sanctioned this session painted them into a corner if they do not comply next session,” he said.

In 2012, in the lawsuit known as McCleary, the Supreme Court ruled that the state government is violating the state constitution by not fully funding a program of basic education for elementary and secondary students. Compliance could require the state spend as much as $4 billion more each year than it now does on schools.

The court gave lawmakers and the governor until the start of the 2017-18 school year to comply.

Justices also demanded regular progress reports. In January, concerned by a lack of progress, they demanded that lawmakers provide a “phase-in schedule” for meeting the 2018 deadline. That plan was due April 30, but lawmakers did not turn one in.

On Sept. 3, the court conducted a hearing on whether to find the Legislature in contempt and whether to impose sanctions such as a fine, a ban on spending for non-education services or even the invalidation of tax breaks.

In the hearing last week, Senior Assistant Attorney General Alan Copsey argued against sanctions but asked that if the court settles on a punishment, it not be imposed until after the 2015 session. Lawmakers convene their 105-day session on Jan. 12.

Lawmakers understand the seriousness of the situation and are motivated to make decisions in the upcoming session to ensure they meet the deadline, Copsey said.

He cautioned that some sanctions might not be constitutional because they would usurp the authority of the Legislature.

In the ruling, Madsen rebutted the state’s claim that the court “may be approaching its constitutional bounds” and venturing into political and policy matters reserved for the Legislature.

She wrote the court doesn’t want to “dictate” how lawmakers carry out their responsibility or want to “directly involve itself in the choices and trade-offs that are uniquely within the legislature’s purview.”

By issuing the contempt order, the court has “fulfilled its constitutional role to determine whether the state is violating constitutional commands,” she wrote.

The next step, the court said, is up to legislators.

Jerry Cornfield: 360-352-8623; jcornfield@heraldnet.com.