hile being treated for leukemia seven years ago, Kim Tolnar, a 1983 graduate of River Valley High School near Marion, Ohio, was contacted by another woman who had attended the same school in the late 1980's and was battling the same disease.

The two young women soon discovered nine other cases of leukemia among the more than 5,500 students who had attended the school since it opened in 1963. The number of cases in a population that size over that period would ordinarily be expected to be three, statisticians said.

Though district officials knew the school had been built on the site of an Army depot, not until the leukemia cases surfaced did they learn that part of the site had been an Army dump for solvents and automotive lubricants.

The story of River Valley and the arguments for and against closing it are in 1 of 15 case studies in a report to be released today by a coalition of parent and community activists known as the Child Proofing Our Communities Campaign. The report's authors said that serious health problems reported by graduates of schools built on old landfills and factory sites were increasing but that districts still relied on such sites for schools.

In describing the appeal of such locations, school districts cite the pressures of escalating enrollments and real estate prices, and they argue that there is little cause for concern unless a link between the chemicals on a site and a cluster of cancers can be confirmed. Districts also say that clusters of cancer cases can simply be statistical aberrations.

Dr. Philip J. Landrigan, the director of the Center for Children's Health and the Environment at Mount Sinai School of Medicine in New York, who was not involved in the study, suggested that districts should err on the side of children when chemicals were involved.

"I don't think anyone would disagree with the proposition that children should go to school in a safe place," Dr. Landrigan said. "The critical issue is where you set the threshold.

"The way in which regulation works in most circumstances is that chemicals are considered innocent until proven guilty," he added. "But when there's a preponderance of evidence, I think it's probably more reasonable to act."

At River Valley, in a rural community of Marion County about 45 miles north of Columbus, about half of the school's 78 acres have been fenced off, including several ball fields where chemicals known or suspected to cause cancer were detected.

State and district officials contend that the school's occupants face no risks, so River Valley High will remain on the site until at least 2003, when a new school is to be built. But people who believe they are victims of the location are not pleased.

"It infuriates me that there are kids right now going to school there," said Ms. Tolnar, 36, whose cancer is in remission. "There are parents who know about what happened to us, and they still don't get it."

Among the other cases examined in the report was that of a high school in Elmira, N.Y., where 22 cases of cancer, including testicular cancer, were confirmed among 7,500 people who attended in the last two decades. The land on which the school was built had been used for industrial purposes as far back as the Civil War. State officials have declared the school safe, but parents dispute those findings and want it closed.

Trying to rebut the arguments of school districts that consider former industrial sites cost-effective, the report's authors recount the travails of the Los Angeles school system. As early as 1993, state officials warned the school district about the dangers of building a high school on an abandoned oil field, but their concerns went unheeded. Last year, after spending more than $125 million on the project, the district scrapped it, largely because methane was seeping out of the soil.

"You could look at what happened at Love Canal and say, `We didn't know there was a risk,' " said Lois Gibbs, whose family was among hundreds of people who fled that upstate New York community two decades ago after learning that it had been built on a toxic dump.

Ms. Gibbs, the executive director of the Center for Health, Environment and Justice, a nonprofit organization in Washington, helped organize the study to be released today.

That such cases can be complicated is underscored by the continuing fight over River Valley High.

Tests have revealed elevated levels of benzo(a)pyrene, a carcinogen similar to the tar in cigarettes, and tricholorethylene, a widely used solvent that might be carcinogenic, in the soil around the school. But state health and environmental officials have said there is no way for students to ingest or inhale those chemicals, particularly with the ball fields closed as a precaution.

"Contaminated doesn't mean it's dangerous," said Thomas G. Shade, the superintendent of the River Valley school district, who supports the school's continued operation. "It just means it's contaminated."