After several years of state and local budget cuts, thousands of school districts across the nation are gutting summer-school programs, cramming classes into four-day weeks or lopping days off the school year, even though virtually everyone involved in education agrees that American students need more instruction time.

Los Angeles slashed its budget for summer classes to $3 million from $18 million last year, while Philadelphia, Milwaukee and half the school districts in North Carolina have deeply cut their programs or zeroed them out. A scattering of rural districts in New Mexico, Idaho and other states will be closed on Fridays or Mondays come September. And in California, where some 600 of the 1,100 local districts have shortened the calendar by up to five days over the past two years, lawmakers last week authorized them to cut seven days more if budgets get tighter.

“Instead of increasing school time, in a lot of cases we’ve been pushing back against efforts to shorten not just the school day but the week and year,” said Justin Hamilton, a spokesman for the federal Department of Education. “We’re trying to prevent what exists now from shrinking even further.”

For two decades, advocates have been working to modernize the nation’s traditional 180-day school calendar, saying that the languid summers evoked in “To Kill a Mockingbird” have a pernicious underside: each fall, many students — especially those who are poor — return to school having forgotten much of what they learned the previous year. The Obama administration picked up the mantra: at his 2009 confirmation hearing, Secretary of Education Arne Duncan declared, “Our school day is too short, our school week is too short, our school year is too short,” but its efforts in this realm have not been as successful as other initiatives.