DALLAS (AP) -- Texas' complicated school finance system is constitutional, the state Supreme Court unanimously ruled Friday - a surprise defeat for the 600-plus school districts that endured more than four years of costly legal battles hoping judges would force the Republican-controlled Legislature to fork over more funding.

The all-Republican court reversed a lower judge's decision that had sided with schools and called state lawmakers' $5.4 billion in classroom cuts in 2011 inadequate and unfairly distributed among the wealthy and poor districts.

The 9-0 decision ends a case that was the largest of its kind in Texas history. Major legal battles over classroom funding have raged six times since 1984, but the latest ruling marks just the second time that justices have failed to find the system unconstitutional. It also means the Texas Legislature won't have to devise a new funding system.

"Our Byzantine school funding 'system' is undeniably imperfect, with immense room for improvement. But it satisfies minimum constitutional requirements," the court found in its ruling. "Accordingly, we decline to usurp legislative authority."

The court also said "there doubtless exist innovative reform measures to make Texas schools more accountable and efficient, both quantitatively and qualitatively" but it added that "our judicial responsibility is not to second-guess or micromanage Texas education policy."

The school funding mechanism is a "Robin Hood" formula where wealthy school districts share local property tax revenue with districts in poorer areas. Districts rely heavily on property taxes because Texas has no state income tax.

School districts in all parts of Texas were on the same side in the case. While those in economically challenged areas said funding was inadequate, districts in well-to-do locales argued that voters often refuse to approve local tax increases because much of the money would go elsewhere.

Texas State Teachers Association President Noel Candelaria said in a statement: "It is a sad day when the state's highest court decides that doing the least the state can do to educate our children is enough."

Lt. Gov. Dan Patrick, the leader of the Texas Senate and former head of its powerful education committee, admitted that "Robin Hood' doesn't work well" and that lawmakers would continue making improvements to school funding.

But with the court fight over, the pressure is off.

"The school funding issue, for now, has been resolved," Patrick said at the Texas Republican Convention in Dallas. "The Supreme Court said we're right."

At issue were the massive cuts to public education and related classroom grant programs that the Legislature approved in 2011, when the state's economy was still reeling from the Great Recession. That prompted more than 600 rich and poor school districts - which educate three-quarters of the state's public school students - to sue, arguing they could no longer properly function amid Texas' public school enrollment growth of nearly 80,000 students annually.

Exacerbating the problems, the districts argued, was the Legislature's increased demand for student and teacher accountability as measured by standardized testing scores and tough curriculum standards.

One of the groups suing was the Mexican American Legal Defense and Educational Fund, which pointed to the growing number of Texas students who need extra instruction to learn English.

"It is incredibly disappointing that a system so deeply flawed could be interpreted as passing constitutional muster," said Democratic State Rep. Ana Hernandez, legal counsel to the Mexican American Legislative Caucus. "The 'imperfections' cited by the Court in its decision are no small wrinkle. They have profound consequences for urban and rural school districts serving low-income communities."

Democratic Texas District Judge John Dietz's first ruling in 2013 found the state's system didn't meet the Texas Constitution's requirements for a fair and efficient system providing a "general diffusion of knowledge." State lawmakers responded by restoring more than $3 billion to schools and cutting the number of standardized tests required for high school students to graduate from a nation-high 15 to five.

Dietz reopened the case to hear how that would impact schools, but didn't change his mind before issuing his written ruling in August 2014, which the state appealed to the Texas Supreme Court. Last year, the Legislature pumped about another $1.5 billion into schools, but that wasn't enough to cover the 2011 cuts when adjusted for enrollment growth and inflation.

When lawmakers reconvene in January, there will be no court-mandated funding limits.