Upon Mr. Buck's retirement in 1943 the school went on a war footing. Depleted by the call to arms, Berkshire nonetheless remained in session throughout the calendar year, with small groups graduating every twelve weeks. Many of the students became seasoned pilots through the school's Education with Wings program at the nearby Great Barrington Airport.
Headmaster John E. Godman, led the school from 1951 to 1970. During his tenure, enrollment increased to 330 boys and the faculty expanded to 35 teachers, among them the school's first female faculty members. By 1970, there were 16 major new structures, six of them dormitories, as well as a laboratory science wing added to Berkshire Hall.
In 1969, near the end of Mr. Godman's leadership, Berkshire School become coeducational. In the fall of that year, nine girls (all day students) took part in what Mr. Godman described to them and their families as "an experiment." The school graduated its first female in 1972.
In the 1970s and 1980s the school restructured its scholastic mission by adding new disciplines, including computer science, ethics, health and environmental science, as well as a formal counseling program. A new, 38,000-square-foot athletic center was built and the former gymnasium renovated to house a modern library which today boasts over 40,000 volumes.
The 1990s saw the construction of a new student center and dining hall. Also in that decade, the curriculum broadened further and innovative academic and co-curricular programs were introduced, including the Ritt Kellogg Mountain Program, a Chinese language program, and a leadership and character development program.
As a result of the school's Berkshire 2000 capital campaign, three faculty chairs were established, two new girls' dormitories were built, dorms and classrooms were fully wired, and the school's endowment nearly doubled. Other improvements included a computer-controlled observatory, an all-weather track, a new maple syrup house, and a renovated hockey rink.
On December 21 2000, a sexual harassment lawsuit was filed against the school. An employee claimed that then-Head of School Paul Christopher had sexually harassed her in his office. This incident resulted in extensive media coverage. Christopher resigned in June 2003.