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.