Merrill encouraged Commonwealth students to be "decent, socially responsible, generous people," actively engaged in public affairs.

Merrill insisted that the school has only one rule: "No rollerskating in the halls,"—an exhortation that students should not "act like a damn fool, but think about your actions and how they affect others.

[3] There have been six heads of school:[5] Commonwealth is commonly compared to a liberal arts college or university in its curriculum.

[6] Due to the small physical size of the school building, it is unable to host any recreational athletic facilities, instead using the "Cafegymnatorium" in the lower level, next to the kitchen, for all-school gatherings and meetings.

[9] Because of the school's location in downtown Boston, it is very accessible by public transit, so for students who do not drive or live in the vicinity, many bike or take the T. With a short walking distance, Commonwealth is directly serviced by light rail through the Green Line at Copley station (between Arlington and Hynes) or by the Commuter Rail and Orange Line at Back Bay station.

Instead, students treat nearby Back Bay as its campus and often visit many local attractions during free periods without classes, lunch, or after school.

[12] Students are the primary organizers of clubs and all-school events, and are expected to dedicate considerable time to community service and studying under the advisory of a professional mentor each year.

"Once endowed, the schools are empowered to perpetually fund scholarships to motivated top students based on merit and financial need.

"[16] While Commonwealth is not officially a "feeder school," many of its students enroll in prestigious colleges and/or universities after graduating.

Popular college choices among graduates from the past ten years include Brown, Bryn Mawr, the University of Chicago, Columbia, Cornell, Harvard, Haverford, Johns Hopkins, New York University, Northeastern, Northwestern, Princeton, Smith, Tufts, UMass Amherst, Wesleyan, and Yale.

[18] Speakers have included Archbishop Desmond Tutu, Ted Sorensen, literature critic James Wood, author Claire Messud, Michael Kelly of the Atlantic, Harvard Law professors Charles Fried and Lani Guinier, author Samantha Power, Harvard stem cell biologist Douglas Melton, philosopher and bioethicist Frances Kamm, poet Louise Glück, former ambassadors Peter W. Galbraith and Charles Stith, Mary Beth Cahill, American feminist and civil rights activist Peggy McIntosh, free software pioneer Richard Stallman, Congressman Barney Frank, Senator Elizabeth Warren, Boston mayor Michelle Wu, former Surgeon General C. Everett Koop, Boston city councilor Tito Jackson and various musical groups.

While the Cafegymnatorium has a singular basketball hoop and its annex has ping-pong tables, it is not large enough for students to exercise or play sports, and instead, the school uses local facilities in downtown Boston, such as the East Boston YMCA or Tennis & Racquet Club for student sports.

[19] Most recently in 2022, the Commonwealth Boys Varsity soccer team won the MBIL Division D Championship.

Each day includes "Recess," when all students and faculty gather in the school's multi-purpose "Cafegymnatorium" for a snack and to hear announcements.