Using data from the transcripts of 1,700 students who entered four high schools as ninth graders in fall 1979, we estimated the influence of students' background characteristics on course placement in mathematics and science. After controlling for the students' background characteristics, we found that course-placement probabilities differ substantially across the four schools. Furthermore, the pattern of association between course placement in mathematics and in science differs in important ways from school to school. These differences in course placement can be explained, in part, by the organization of the curricula at the four schools and, in part, by the composition of the student populations in the schools. The results suggest that stratification in high schools is a dual process in which positions in the curriculum (courses or sections) are created and students are matched with positions.