In courses with a heterogeneous student population, instructors are often challenged to balance successful course completion with rigor. This difficult task can be confounded in foundational, gateway courses, such as introductory biology, which serves a mix of freshman majors at various levels of preparedness. Research suggests that changes in course design, such as increasing course structure, can offer a solution. We hypothesize that increased course structure enables instructors to increase exam rigor without coincidentally increasing failure rates. Sixteen sections of general biology classes over the course of eight semesters were analyzed; eight sections had relatively low class structure (i.e., mostly Socratic learning and clickers), while eight sections had moderate structure (i.e., including class note summaries and practice exams). Weighted Bloom's Index of 150 exam questions was used to facilitate comparisons between designs. Although exam rigor increased (as gauged by WBI), in moderate structured courses, student exam scores, perception of the difficulty of the subject matter and failure rates did not change in comparison to low structured courses. This study supports the use of increased course structure to balance student success and rigor. Additionally, it supports the use of Weighted Bloom's Index as a method for assessing exam equivalence across institutions.