Large-scale externally mandated exit exams are common in many education systems. Exit exams are considered medium stakes when exam scores are blended with school-awarded marks to determine a final course grade. This study examined the effects of policy decisions regarding the weighting of exam and school-awarded marks when calculating a student's final blended grade. Grade 12 teachers (n = 343) in the Sciences and Humanities were surveyed regarding the effects of Alberta's exit exam program on their planning and assessment practices. This study found that these exit exams had a profound impact on teacher practice at both the 50% and 30% weightings, leading to narrowing of planning and assessment practices. While a recent change in weighting contributed to a minor shift away from this practice, perceived narrowness of the exit exam's content sample and perceived pressure related to classroom assessment outcomes were factors that mitigated this potential shift.