The Illinois Supreme Court yesterday declared unconstitutional an ambitious school-reform effort designed to improve the foundering Chicago school system, ruling that the structure of the plan to give parents more control over local schools violated the constitutional requirement of one-person, one-vote.

The reform plan was viewed as the most revolutionary attempt in decades to revamp a troubled big-city school system, shifting control from a large central bureaucracy to communities in an effort to improve a system that then-Education Secretary William J. Bennett labeled the nation's worst.

The plan, passed overwhelmingly by the Illinois legislature in 1988, established local school councils for each of the city's 539 grammar and high schools, with authority to approve budgets, design currricula, choose books and hire and fire principals. Each council has 10 elected members -- six parents with students in the school, two community representatives and two teachers at the school.

Striking down the plan in a 43-page opinion, the court said the method for electing council members, in which parents of school children had more clout than other voters, violated the constitutional requirement of one-person, one-vote.

Supporters of the plan said yesterday that changes could be made that would satisfy the constitutional requirements and maintain community control of the nation's third-largest school system.

"Chicago school reform will remain intact and move forward," state Sen. Arthur Berman, a sponsor of the act, told the Associated Press. "We can broaden the election participation, and that would cure the fault the Supreme Court finds."

A group of Chicago principals and voters challenged the constitutionality of alloting parents six council votes and other community residents two. They also argued that, by eliminating tenure for principals and substituting four-year contracts, the plan unconstitutionally infringed contract rights.

The court rejected the latter claim but voted, 6 to 1, that U.S. Supreme Court rulings make clear that the greater weight given to parents violated rights of other voters, who also have a strong interest in effective schools.

"The benefits resulting from the election of competent and efficient local school councils are far from limited to parents with children in the public schools," the court said in an opinion by Justice Daniel Ward. "A school is not an island within the community; the school system is an integral part of the whole city."

It said the entire act was unconstitutional because the local school councils were an integral part of the plan.

In a dissenting opinion, Justice William G. Clark said that there was a rational basis for treating parents and other voters differently and that the majority was incorrect in assessing the plan under a more demanding constitutional standard.

Clark described the law as "a proper and constitutional response to the educational crisis facing the city of Chicago" and said the court should not interfere with legislators' efforts to improve the "dire condition" of Chicago schools unless the law is clearly unconstitutional.

"I do not see this as a big blow to school reform," said Joan Slay, acting director of Designs for Change and an architect of the plan. "I see this as an important but a technical change that needs to be made."

State Senate President Philip Rock told the Associated Press that lawmakers would consider the issue in the spring after consulting with Chicago city and school leaders.