This research adds to the growing body of music self-efficacy literature by profiling changes in instrumental performance self-efficacy perceptions of 157 high school student musicians over the course of a 3-day competitive honor orchestra festival in order to clarify contextual influences upon self-efficacy perceptions within a high-stakes music performance environment. Student participants completed surveys, participated in interviews, and were observed by a team of researchers over the course of the festival. Reported instrumental performance self-efficacy beliefs were profiled over time by characteristics of gender, orchestra placement, and influence of competitive environment versus social support. Repeated measures analysis revealed a significant general increase in students’ instrumental performance self-efficacy beliefs over time, with a delayed increase in self-efficacy beliefs of females placed in the top orchestra. Qualitative data from interviews, observations, and open-ended survey responses were analyzed according to the sources of self-efficacy and in relation to statistical results. Findings suggest a strong influence of enactive mastery experience for all students, and a negative influence of competitive environment upon female self-beliefs. Recommendations include providing positive enactive mastery experiences for all students, and teacher awareness of student differences by gender, competitive placement, and influence of social support.