In a union vote Wednesday, Boston teachers approved the school district’s plan to add 40 minutes to each instructional day for kids in grades kindergarten through eight at more than 50 campuses. It's a move experts say could help improve the quality of classroom teaching, boost student learning, and yield long-term benefits to the wider community.

But the plan, which goes next to the Boston school board for approval, isn’t without controversy. Earlier in the week The Boston Globe published its own review of a pilot program in the city that expanded learning time at about 40 campuses, finding mixed results. From the Globe’s story:

For many schools, a longer day has failed to dramatically boost academic achievement or did so only temporarily. The uneven results prompted school district officials to scrap the extra minutes at some schools and the state to pull funding or pursue receiverships at others. But other schools have successfully used an extended day to boost MCAS scores or expand offerings in the arts and other electives. "I think there are lessons to be learned," said John McDonough, interim superintendent. "We know time matters, but it only matters if it is used well."

The idea that the typical American school day is too short is far from a new idea in education circles. But it's certainly gaining popularity, backed by recent studies that have found improvements in student performance when kids spend more time in school. However, ongoing tensions remain over whether American schools need longer days—or, rather, strategies for being more productive with the time already allocated. As with many debates about education, the answer probably falls somewhere between those two extremes.

American students spend an average of about six-and-a-half hours at school each day over a 180-day calendar. In Boston, for example, the typical school day is six hours for elementary students, with middle schoolers staying in class an extra 10 minutes. The city’s high schoolers average a six-and-a-half-hour day.