Hundreds of thousands of children are at risk of developing asthma, cancer, learning disorders, and other diseases because they attend schools built on or near toxic waste sites, according to a new study released yesterday by a coalition called the Child Proofing Our Communities Campaign. The study found that most states and public school systems do not have environmental standards for selecting school locations; as a consequence, cash-strapped districts often choose to build schools on cheap land near environmental hazards. In the five states studied, 600,000 students were attending public schools located within a half-mile of Superfund sites or state-identified toxic waste areas. The study correlated that finding with the sharp increase in environmentally linked diseases among school children in the last two decades.