Millions of at-risk students could fall through the cracks as the Education Department gives states permission to ignore parts of No Child Left Behind, according to a study released Tuesday. The Education Department has been giving some states waivers from the law’s requirements, including those to collect and publish data about students and then use the results to pinpoint problem schools. The resulting patchwork of rules — from Miami to Seattle — has given states more freedom to carry out plans to boost education but has allowed almost 2,300 schools to shed their label of seriously troubled, according to numbers compiled at the Campaign for High School Equity.