NEW YORK, JUNE 5 -- The New Jersey Supreme Court struck down the state's school-finance system today as so reliant on local property taxes that it unconstitutionally penalizes poorer urban districts.

The ruling gives an important boost to a plan proposed last week by Gov. James J. Florio (D) that would cut state aid to many affluent school districts and raise income taxes on residents earning more than $100,000. The plan has drawn angry protests from the richest districts, some of which would lose all school aid from Trenton.

The court, one of the most liberal in the country, estimated that its ruling would have added $440 million to this year's $3.5 billion state education budget to remedy "extreme disadvantages" faced by students in cities such as Newark, Jersey City, Paterson and Camden. The court gave the legislature until September 1991 to fix the problem.

Florio hailed the ruling as "a clear victory," saying: "The court has recognized that the current school funding system is leading us to be two New Jerseys, where some youngsters head for the best colleges and others go down a road to nowhere." His plan would boost school aid to poorer cities by as much as 40 percent.

Courts in a dozen states have overturned school-finance systems in recent years. In Texas, Gov. Bill Clements (R) and the legislature have been wrangling for months about a sales-tax increase that would fund education improvements.

The New Jersey court has been grappling with the issue since 1975, when it essentially ordered the legislature to adopt an income tax to fund schools. Today's ruling, based on the state constitution, came in a 1981 lawsuit that had been fought by former governor Thomas H. Kean (R).

The unanimous ruling noted that "Princeton has seven laboratories in its high school, each with built-in equipment. . . . However, many poorer urban districts offer science labs built in the 1920s and 1930s, where sinks do not work. . . . South Brunswick offers music classes starting in kindergarten. . . . In 1981, Camden eliminated all its elementary school music teachers."

The justices rejected the idea of setting minimum aid for poor districts. Instead, they said, the state must help poorer schools to match the average expenditure of the wealthiest districts and must do it without forcing an increase in local property taxes.

Marilyn Morheuser of the Newark-based Education Law Center, which filed the suit on behalf of 20 schoolchildren, called the ruling "a revolutionary decision" recognizing that poor children "don't start out on a level playing field."

She said that the Florio plan "falls short" by giving affluent districts too much latitude to increase school spending on their own, but that the ruling would remedy this by requiring increased aid to poorer districts as the statewide average rises.