School-based mentoring programs are one of the most popular types of prevention programs offered to support students' social-emotional and academic success. Because a high-quality mentoring relationship is thought to be a mechanism for improving youth outcomes, we investigated how mentor- and mentee-relationship quality develops over time. In addition, we examined how relationship development varied as a function of three approaches to selecting mentoring activities (i.e., mentor-directed, program-directed, or jointly determined). We used a random-intercepts crossed-lagged panel model to test how mentor and mentee reports of relationship quality developed over three time points across one school year. Data were gathered from 2014 to 2018 and included 47,699 youth (56% female; 51% Black or Hispanic) who participated in Big Brothers Big Sisters-which provides school-based mentoring services to youth across the U.S. Results indicated that mentors' positive appraisals of the mentoring relationships led to positive appraisals from the mentee at the subsequent time point. In addition, when mentors and mentees jointly determined activities, we found evidence that mentors' and mentees' positive appraisals at one time point led to positive appraisals at the following time point. These results suggest that school-based mentoring programs should consider ways to support joint decision-making among mentors and mentees participating in school-based mentoring programs. (PsycInfo Database Record (c) 2021 APA, all rights reserved).