(Marvin Joseph/The Washington Post)

Jennifer L. Steele is an associate professor of education at American University and studies education economics and policy.

Maryland Gov. Larry Hogan (R) recently issued an executive order that will lengthen summer vacation for nearly every school district in Maryland. It requires the state’s public schools not only to start the school year after Labor Day — the first Monday in September — but also to end by June 15. The move, which has previously been rejected by the General Assembly, is aimed at stimulating the state’s summer tourism industry. But it may do so at the cost of student achievement, especially for the state’s most vulnerable students.

Like many states, Maryland requires public schools to provide at least 180 instructional days per school year, and the executive order doesn’t change that. But now districts will have to allocate almost 12 weeks of each year for summer vacation, which is one to two weeks more than the status quo. For districts that currently exceed the 180-day minimum, such as Baltimore County with 188 days and Montgomery County with 184, the order may be especially onerous, requiring that they request a state waiver or shorten the instructional year.

The big question is how the order will affect student learning. Decades of research have shown that students forget some of their learning during the summer, especially in math — a phenomenon known as summer slide. Worse, the size of the slide depends on students’ socioeconomic backgrounds, with those from low-income backgrounds losing more ground, especially in reading. Why? One factor may be that wealthier students have greater access to enriching summer activities — camps, travel and internships. They may also spend more time on literacy-related tasks.

In fact, some of the best evidence about summer learning disparities comes from Baltimore City. A team of Johns Hopkins University sociologists found that differences in students’ summer learning between kindergarten and fifth grade explained two-thirds of the reading achievement gap between high- and low-income students in ninth grade. It also predicted at least a quarter of the gap in high school completion and at least half the gap in college entry. In other words, summer matters.

To be fair, the executive order will force most districts not to shorten their academic years but to compress them, by slashing teacher planning days and midyear breaks. Though the research on compressing vs. expanding the instructional calendar is mixed, there is at least some evidence that distributing breaks at regular intervals across the year may benefit the lowest-income students.

The state has estimated that the longer summer will add $74 million a year to the tourism economy and more than $5 million in state tax revenue. But the latter figure is dwarfed by the $6.3 billion the state will spend on K-12 public education in 2017, and it would yield only about $40 in new state tax dollars for each school-age child living in poverty in Maryland. It is not nearly enough to fund a statewide summer enrichment program for low-income youths, but perhaps the revenue, if it materializes, could finance seed grants to support local summer programs.

The takeaway? When it comes to growing the Maryland economy, forcing districts to compress the school year may be penny-wise and pound-foolish. Lengthening summer vacation will not help and may hurt student learning, especially for the children whose success depends the most on their schools. For a long-term economic boost, the governor should put education first.