When schools let out across Mississippi in May, more than 450,000 children headed into 11 weeks of summer. For some kids, that means weeks of camps, traveling, and educational programs. For others, especially in the most poor and rural parts of the state, summer means hours of lost learning time, unsupervised activities, and academic regression.

Extensive research shows that all children are prone to learning losses when they do not have educational opportunities during the summer. Some studies have found that, on average, students return to school in the fall about one month behind where they were academically before summer. But for low-income students, these losses can be much more drastic as higher-income kids improve their reading over the summer, while lower-income students lose skills. One study found that this could lead to a three-month difference in reading skills by the end of the summer.

In Mississippi, black students suffer most from summer learning loss. More than 50 percent of black children in the state live in poverty, compared with about 19 percent of white children. Boys in Mississippi are more likely to start the summers already behind grade level.

In 2013, only 55 percent of fourth-grade boys scored at or above proficient on the state's end-of-year reading exam compared with 64 percent of girls. The difference is even more glaring in some of the state's poorest school districts. In the Delta's Coahoma County School District, where 90 percent of students are black and 50 percent live in poverty, only 36 percent of fourth grade boys scored at or above proficient on the state reading exam in 2013, compared to 71 percent of girls.

Research shows that the most effective summer programs have similar characteristics, like high-quality instructors and lessons that introduce students to new content. In programs nationwide with those qualities, students don't just avoid learning loss; they often show months of growth in academic skills.

The benefits of summer learning are so evident that many cities have ramped up summer-school offerings. This year, the Los Angeles Unified School District in partnership with the city will offer free classes in science, technology, engineering, arts, and math to residents between the ages of three and 24. In previous summers, Chicago has enrolled nearly 15,000 students in remedial summer school and relies on partnerships with more than 100 organizations to provide a variety of summer opportunities. Some programs specifically target young men of color and offer both summer-school programs and extended-day opportunities during the school year.

In Mississippi, opportunities for summer learning vary greatly by district.

In larger districts like Jackson Public Schools, summer-school options target a range of students for at least half of the summer. The district's middle-school program, which costs $250, offers six weeks of classes for students who are failing one or two courses and at risk for retention. One free district program works with second-grade students who need to "strengthen and extend skills not mastered during the regular school year."

In the Delta, Teach For America, a program that trains aspiring teachers who have not taken traditional teacher preparation classes, has offered summer school since 2010 in conjunction with its teacher training program. This year, the organization will run summer-school programs for four weeks at nine schools.

Throughout the state, it isn't easy for the smallest districts to offer summer opportunities. Mississippi has underfunded its schools by more than $1 billion in the past six years, and many districts are reeling from staff cuts and a lack of resources.

In Water Valley, a small district of about 1,200 students just south of Oxford, Superintendent Kim Chrestman said the district offers an extended school year in lieu of traditional summer school. The program targets upper elementary and junior high students who are "borderline or just below" grade level and runs several weeks into the summer to help students earn credits that they have missed so they can progress to the next grade with their classmates.

Although grants are sometimes available for summer-school programs, Chrestman said there are often additional costs to the districts, which can deter cash-strapped schools. In Water Valley, about 70 percent of students qualify for free or reduced lunch, and Chrestman said the current summer program is essential to keeping the most at-risk kids progressing in school.

"If there was not a summer program, there's a likelihood they would have to repeat the grade level, and they would get further behind toward graduation."

This story was produced by The Hechinger Report, a nonprofit, nonpartisan news outlet based at Teachers College, Columbia University.