New York ? The lazy days of summer make for hazy days in the classroom when a new school year begins.

With summer vacations over or winding down across the nation, education experts say teachers will spend the first four to six weeks of the new school year simply rehashing material that their young charges learned in the previous school year but forgot over the summer.

It’s a phenomenon so widespread and well-recognized that it even has a name: the summer slide.

“Research confirms what most people accept as common sense, which is that if you don’t practice something, you suffer a loss,” said Ron Fairchild, executive director of the Center for Summer Learning at Johns Hopkins University.

Studies have found that students in all income groups fall an average of 2.6 months behind in math skills, possibly because few students are likely to practice much outside the classroom.

But when it comes to the summer slide in reading, household income is an important factor. Children in low-income households lose an average of two months in reading ability, while their middle- and upper-income counterparts tend to make slight gains in reading levels over the summer months.

Researchers attribute that difference in part to the greater opportunities that children in more affluent households have to participate in costly summer programs such as specialized camps, or to go with their parents on educational vacations that keep their minds stimulated.