To determine if there are critical differences between the social and psychological environments of coeducational and single-sex schools, researchers investigated five coeducational, four all-female, and four all-male high schools in Ontario (Canada). All of the schools are "Separate" schools; that is, they are Roman Catholic and, therefore, are attended primarily by students of that religious background. A total of 2,929 grades 10 and 12 students completed measures of value climate and "environmental press" (defined as the pressures, stresses, rewards, and conformity-demanding influences of the culture, and measured here by the High School Characteristics Index). The hypotheses evaluated were that coeducational schools, compared with single-sex schools, would be perceived by their students as placing (1) less emphasis on scholarship and achievement, (2) greater emphasis on affiliation and pleasurable, nonacademic activities, and (3) less emphasis on control and discipline. Support was attained for the second and third hypotheses, but the evidence relevant to the first hypothesis was inconsistent. The results suggest that coeducational schools enjoy an advantage in that they attend to social-emotional needs and minimize the necessity of regimentation and discipline, and that this advantage does not occur at the expense of academic concerns. (Author/MLF)