The school day and year are about to get longer in 10 school districts in five states, where schools will add up to 300 hours to their calendars starting next fall.

In an effort to help underperforming students catch up on standardized tests and give them more opportunities for enrichment activities, 35 schools that enroll about 17,500 students will expand the school day and year in the 2013-14 academic year. Forty more schools that enroll about 20,000 students will also extend classroom and after-school time in the next three years.

The effort is being coordinated by state education officials; the National Center on Time and Learning, a nonprofit research and advocacy group; and the Ford Foundation, which is committing $3 million a year in grants over the next three years. The districts will use state and federal financing to pay for all of the operating costs, including extra teaching time and coordination with nonprofit groups.

Already, more than 1,000 public schools across the country, including numerous charter schools, have added more time to the school day and year. A growing group of education advocates is pushing for schools to keep students on campus longer, arguing that low-income children in particular need more time to catch up as schools face increasing pressure to improve student test scores. Advocates also say that poor students tend to have less structured time outside school, without the privilege of classes and extracurricular activities that middle-class and affluent children frequently enjoy.