Retrieving information from memory increases the likelihood the information will be remembered later. The strategic use of retrieval to enhance memory is known as retrieval practice. Teachers can exert considerable control over students' retrieval practice, dictating when and how much students practice. Laboratory research has shown that retention benefits from increasing the amount of practice (i.e., the number of times information is retrieved) and from spacing practice out over time. Although retrieval practice is a prominent part of the learning experience in certain educational domains, such as mathematics, relatively little research has examined how retention of actual classroom content is affected by increasing the amount and spacing of retrieval practice. Here, we implemented a complete within-subjects crossing of practice amount (baseline versus increased) and practice spacing (baseline versus increased) in a precalculus course for engineering students. Practice consisted of answering quiz questions. We assessed retention of precalculus knowledge at two educationally relevant time points: the end of the precalculus course (within-semester) and the beginning of a calculus course 4 weeks later (across-semester). Within-semester retention benefited significantly from practicing more and from spacing out practice, although some evidence suggested that the effect of amount of practice was less robust than the effect of spacing. Across-semester retention benefited exclusively from increasing spacing. Given that retaining precalculus knowledge across semesters is crucial for success in higher-level mathematics, these findings support increasing spacing in real-world mathematics education. We discuss how our findings fit within the larger literature on the memory-enhancing effects of retrieval practice.