In a finding that will shock absolutely no parent (and any rationale human being), a new review of 74 scientific studies has concluded that students who spend more time in school tend to score higher on tests than students who spend less time in school.

The University of Oregon’s HEDCO Institute released the summary (PDF File), synthesizing work by Brown’s Matthew Kraft and Stanford’s Sarah Novicoff. Somewhere, a school board member is updating a PowerPoint slide…

The sarcasm aside, the actual numbers matter — because they tell policymakers what kind of “more time” is worth paying for, and what isn’t. Because simply adding time to add time isn’t what’s helpful. It’s adding time in core areas.

Why it matters: State and district leaders are still wrestling with pandemic-era achievement gaps, four-day school week experiments, and proposals to extend the school day or year. This review gives them a cleaner read on which interventions actually move the needle and which ones are a rounding error.

By The Numbers

In 74 studies reviewed by the researchers, they found:

• A 10% increase in total time in school is the threshold most likely to produce measurable gains.
• Schools that stacked multiple changes (longer day, longer year, tutoring) saw math gains of 0.09 to 0.38 standard deviations, moving a median student from the 50th to as high as the 65th percentile.
• Schools that added just one day to the school year saw math effects of 0.003 to 0.019 SD.
• 31 states require 180+ school days. One state has no minimum.

To compare the United States to the world stage: 

• China: 245 days
• South Korea: 220 days
• Japan: 200 days 
• Singapore: 200 days
• Finland: 190 days

What the study found: Total time in school helps, but the size of the benefit depends heavily on how you add it. Bundled changes work best.

A New York and Massachusetts charter school review and a Texas public school study found the strongest gains came from schools that extended the day, extended the year, and layered in tutoring at the same time.

Single-lever changes underwhelm. Adding one day to the calendar in Colorado, Maryland, and Minnesota barely registered. Extending the day in Massachusetts produced mixed multi-year results, with some grades gaining ground in math and science and others losing ground in ELA.

There are also diminishing returns. Pushing an already-long 7-hour day to 8 hours is unlikely to pay off the way moving a short 5-hour day to 6 hours would.

Cutting time has clearly reduced educational results. Maryland and Massachusetts schools that subtracted a day saw drops in math and reading pass rates and the four-day-school-week trend is not working academically.

How this connects: K-12 time-in-school policy eventually shows up on the college success ledger. 

Students who arrive at college underprepared are more likely to need remedial coursework, take longer to graduate, and rack up additional federal student loan debt (currently a $1.7+ trillion balance nationally). 

Weaker K-12 outcomes also feed into the growing share of high school graduates skipping college entirely.

What to watch next: Watch state legislatures debating four-day weeks (Missouri, Oklahoma, and Colorado have growing numbers of districts on the schedule), districts weighing extended-day pilots with federal grant money, and the next round of NAEP results to see whether time-related interventions are showing up at the national level.

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