One hundred eighty days, 6.5 hours a day, 50-minute classes, 8:30 a.m. to 3 p.m. This schedule has been the norm for high schools for nearly 100 years. But true mastery of 21st-century skills requires a more flexible conception of time. How long should classes be and how often should they meet to make the most effective use of time?

In most high schools, classroom time is a constant, but learning—how much and how well students learn—is variable. Nicholas Donohue, president and CEO of the Nellie Mae Education Foundation, urges us to consider a new paradigm in which students have the time they need to succeed and master truly rigorous content. How might each student’s schedule allow them to fill gaps and accelerate learning, strengthen equity, and deepen students’ engagement in their own learning? How might we make time for advisory and other opportunities to build strong relationships—which have been shown to support learning—among students and between students and adults?

Some schools are rethinking the whole calendar as they look for ways to harness time during the school day, week, or year—including after-school, weekend, and summer hours—to provide more learning time for students who need it, allow students to move more quickly and accomplish highly complex projects, and give teachers time to learn and plan together. At Summit Shasta, for example, an XQ school in Daly City, California, that’s part of the Summit Public Schools network, all students enroll in “expeditions,” or immersive two-week elective courses taught four times each school year.

Schools need flexibility to adjust and vary their schedules so students and educators can engage in deep, sustained learning experiences. Freed from the bounds of a conventional schedule-segmented day, students can immerse themselves more in discussion, engage in individual and group learning, work on interdisciplinary projects, and get field experience applying their skills in the community. At Crosstown High in Memphis, the day is organized into longer and more flexible blocks of time to allow for place-based, out-of-school experiences, not to mention interdisciplinary courses such as humanities/human geography or biology/geometry. Crosstown students also select “flex classes” that give them time to explore or cultivate interests such as filmmaking or podcasting.