Reviewing the Research About Time in School
We start with a wide-ranging search for studies that examine time during the traditional K–12 school day. Our review includes studies that reasonably support causal inferences about the effects of time, excluding those that focus on after-school or summer programs. In terms of outcomes, we focus on academic achievement as measured by test scores. This is a relatively blunt approach on two fronts: not all time in school is active learning time, and time in school can influence a range of student outcomes, such as social-emotional skills and contact with the juvenile justice system. However, total time in school is the most malleable for policymakers, and test scores serve as a common and useful summary metric of students’ academic progress.
In all, we include 74 papers in our review. We grouped the studies and their findings into five categories, discussed separately below.
Increased Time Bundled With Other Reforms
The biggest impacts from longer school days and years are at schools that extend time as part of a larger education reform effort, such as urban charter schools in Massachusetts and those operated by KIPP. Researchers find annual effects of attending these schools in middle school as large as 42 percent of a standard deviation in math and 25 percent in reading. However, these schools also feature tutoring, data-driven instruction, and frequent teacher observations, among other distinguishing features. Several studies attempt to disentangle the component parts and find, to varying degrees, that additional total time predicts larger effects on achievement.
We also find evidence of traditional school districts implementing similar bundled interventions and seeing positive results. In Houston, math achievement increased by 15 percent to 18 percent of a standard deviation at 20 elementary and secondary schools that adopted such reforms, including extending the school day and year to increase instructional time by 21 percent and incorporating high-dosage tutoring throughout the extended school day. In Lawrence, Massachusetts, the state took over the traditional public school district and added 200 hours to the school year for students in grades 1–8, while also increasing school spending, replacing underperforming staff, and adding extra time for underperforming students. Researchers find these reforms increased math achievement by 30 percent of a standard deviation and reading achievement by 10 percent of a standard deviation.
Extending the School Year
Research on the effect of additional school days often seeks to take advantage of unexpected changes in the number of days students are in school before taking standardized tests. These studies tend to show a small positive increase in academic achievement from the addition of 10 or more days before a test. Correspondingly, research finds decreases in the overall academic performance of students who experience an unscheduled loss of total time from snow days, strikes, or other events. Studies looking at optional extended school-year programs also provide some evidence of increases in student achievement, such as from a weeklong math program in Massachusetts, but not all students participate in voluntary programs.
Extending the School Day
Full-day kindergarten offers the best example of a widescale expansion in the length of the school day in the United States, and several studies find almost uniformly positive effects on student learning. For example, in Indiana, students who were randomly assigned to full-day kindergarten outperformed their peers assigned to half-day programs by 31 percent of a standard deviation on an end-of-year assessment of literacy skills. However, studies that examine longer-run outcomes find that these positive effects tend to fade out over time.
Studies of schools in the U.S. that adopted extended days find mixed results. A Massachusetts study found no impact in math, reading, or science after 26 schools added at least 300 instructional hours to the school year. A Florida study found reading scores increased by 5 percent of a standard deviation in a single year after a state policy required the 100 lowest-performing elementary schools to extend the school day by one hour, dedicated to additional literacy instruction.
Studies looking at extending instructional time for specific subjects without lengthening the school day find positive effects on performance in the targeted domain. For example, when Chicago Public Schools required low-performing 9th graders to take two periods of math instead of one, student made gains in math achievement and were more likely to graduate high school and enroll in college years later (see “A Double Dose of Algebra,” research, Winter 2013).
Four-Day School Weeks
In addition to lengthening the school day or school year, districts can also change the way that they structure their existing time, through adjusting academic calendars, class schedules, and the number of school days each week. We find scant causal evidence regarding block schedules and year-round school calendars (see “Busting the Myths About Year-Round School Calendars,” feature, Spring 2023). However, in looking at four-day weeks, research shows that they rarely improve academic outcomes and often lead to decreases in performance (see “The Shrinking School Week,” research, Summer 2021).
Four-day weeks have grown in popularity since the pandemic—teachers like the flexibility these schedules offer, and districts look to their potential to cut HVAC and busing costs. When four-day weeks maintain the same total time in a school year by lengthening the day but reducing the number of days, research shows mixed effects. In Colorado, after small rural districts switched to a four-day week, more elementary students scored proficient on math and reading tests. In Oklahoma, switching to a four-day week was a cost saver and decreased fights and bullying, without negative effects on student achievement.
However, research consistently shows that student achievement suffers when districts shift to a four-day week and reduce the total number of hours in school—the more common scenario. The average student in a four-day school experiences 85 fewer total instructional hours per school year than their counterparts at five-day schools, and research shows a corresponding decrease in math and reading scores. A recent study of student performance across six states on the Measures of Academic Performance (MAP) assessment found student scores dropped by 3 percent of a standard deviation in math and 7 percent of a standard deviation in reading, on average, with larger effects at non-rural schools.
Adjusting School Start Times
While not strictly focused on duration, many states and communities are looking to shift school start times to ensure students are well rested and ready to learn during school hours. Governors in California and Florida have signed new laws mandating later start times, and legislators in 13 other states have introduced bills for consideration.
Later school start times for adolescent students are recommended by the American Academy of Pediatrics and have been shown to improve sleep, mood, and attention while decreasing car accidents. The research shows small to moderate positive effects on state standardized tests when schools delay start times, typically to no earlier than 8:30 a.m. (see “Rise and Shine,” research, Summer 2019). A North Carolina study found that a one-hour delay in middle school start times caused a small increase in math and reading achievement. Studies of a later high school start time in Minnesota show no effects on ACT scores but a positive effect on grades.
