IT TOOK 13 years, but New Jersey's supreme court finally managed last week to declare the state's school finance law unconstitutional. The rejection of the finance system, under which poor districts have spent as little as $3,300 on average per student and rich ones as much as $13,000, is hardly surprising in a year when Kentucky, Montana, Texas and many other states have come to similar conclusions, noting the gaps that result when you pay for education with local property taxes. But the New Jersey decision advances the argument another step. Other state courts have confronted a central question -- is it unconstitutional to fund schools at greatly unequal levels? And in states where those disparities are shockingly large the courts have said that, yes, it is unconstitutional. But they have left it substantially up to legislatures to figure out how and how much to narrow those gaps.

In New Jersey, by contrast, the court didn't say all school income disparities are unconstitutional. What it said, unanimously, was that in a clump of the state's poorest 30 or so urban districts -- already identified as such by the state board of education, which divides school districts into 10 socioeconomic levels -- the level of funding is so low that students can't conceivably be getting the "thorough and efficient" education mandated by the state constitution.

These poorest schools, the court said, are the ones that must get state relief. It instructed the state to bring those 30 urban districts up to the point where average funding per pupil is at least as high as that of the 30 richest, suburban districts -- which are among the nation's plushest. That will cost somewhere above $440 million to do, roughly the price tag on the plan that Gov. James Florio had already been pushing in the legislature. The court scheme leaves alone, for the moment, smaller inequities in mid-range schools; in place of a principle of absolute equality, it merely lays down the principle that a tolerably "equitable" system must give the poorest a fair start.

In specifying certain remedies that the state must also provide -- such as preschool education and individual instruction -- to bring those poorest to the "same starting line," the court stresses that the differences in education between suburban schools and bottom-of-the-barrel ones are of kind as well as degree. That's a crucial point. The awful state of big urban school systems even now -- after seven years of reformist rhetoric -- cannot be overemphasized: violence, lack of parental support, lack of discipline, buildings that are falling down and lack of supplies make even the most elementary tasks difficult, while problems like crack cocaine and family chaos continually enlarge the task. Studies suggest that students in such settings can start off two full years behind in development.

The fashionable policy debate right now is whether money really helps schools most directly. Do they need "accountability" or "restructuring" instead? The New Jersey decision reminds that truly poor urban schools can't even enter that debate until the state hauls them up toward a suburban minimum.