“First base, second base, third base, home run,” Al Vernacchio ticked off the classic baseball terms for sex acts. His goal was to prompt the students in Sexuality and Society — an elective for seniors at the private Friends’ Central School on Philadelphia’s affluent Main Line — to examine the assumptions buried in the venerable metaphor. “Give me some more,” urged the fast-talking 47-year-old, who teaches 9th- and 12th-grade English as well as human sexuality. Arrayed before Vernacchio was a circle of small desks occupied by 22 teenagers, six male and the rest female — a blur of sweatshirts and...