Extended learning time has been at the heart of many of the Obama administration’s school turnaround strategies. Schools that get money through the School Improvement Grant program have to extend the school day, or year. And states with waivers from the No Child Left Behind Act have to add extra learning time for “priority” schools (those that are among the worst in the state) .

But adding extra time to the day or year is a lot easier said than done, according to a report released Tuesday by the Center on Education Policy, a research organization in Washington. And it’s far too early to say whether adding time really has done much to move the needle on student achievement, in part because it’s early going and in part because extended learning time is usually paired with a lot of other strategies.

Some of the implementation issues cited in the report:

Extended learning time is expensive. States that got NCLB waivers were given access to increased resources—control over 20 percent of their Title I funding for disadvantaged kids. And the department encouraged states to spend those dollars on school improvement efforts, including additional learning time. But states studied by CEP didn’t tend to spend that money on adding instructional time. What’s more, states generally choose not to take advantage of another area of flexibility in the waivers: diverting federal money for after-school programs—made available through the 21st Century Community Learning Center grants—to add time to the school day. The reason? It’s generally cheaper to offer after-school programs. And in some cases, adding time the school day meant negotiations with teachers’ unions. Sometimes those discussions were fruitful, and other times less so.

Everyone does extended learning time differently. Some schools added to the school day, others to the school year. And others added instructional time without actually adding any time to the school day or calendar. Some schools used the extra time for instruction, others for teacher professional development and collaboration.

Extended learning time causes fatigue. Longer instructional blocks were hard on both teachers and students. Some schools chose to shift the extra time to teacher collaboration, and others decided to go with enrichment activities or turn the work over to community partners.

So does extended learning time actually work, meaning do student outcomes improve? Tough to say. There seems to be some movement in test scores and graduation rates at schools that added instructional time, but those schools were also trying a package of strategies, so it’s not clear whether the extra time was the decisive factor.