This paper describes the results of a two-semester-long study of the effects of student-centered instruction on Precalculus courses. We also describe our teaching approaches centering around students, which include a mixture of lectures, student presentations, group work, discussion, and guided investigations. Students were taught with either the traditional, and still dominant, lecture-based, instructor-centered teaching methods, or with the collaborative student-centered methods. We found that the average over all students was higher on quizzes when they were taught with student-centered methods. Moreover, the students in the lower quantiles scored significantly higher than their counterparts in the traditional lecture-based classrooms. This suggests that not only are student-centered approaches more effective than instructor-centered approaches, but that they are particularly helpful to those students who are struggling with Precalculus.