The "mentor teacher" within the Professional Development School (PDS) model is in a unique role of providing leadership during the student teacher internship and supporting the development of student interns' efficacious beliefs. Data collected from student intern surveys were used to answer the following research questions. Are student intern efficacious beliefs significantly impacted during the clinical internship within PDSs? To what extent do mentor teacher "adaptive leadership" practices and behaviors have on student intern efficacious beliefs? What can we learn from mentor teachers and student interns that will foster the efficacious beliefs and adaptive leadership qualities of future PDS educators? This study found that student interns grew in their teacher efficacious beliefs within the PDS context. In addition, the study found that adaptive leadership practices of the mentor teacher supported the growth in student intern efficacious beliefs. More specifically, two of the six dimensions of adaptive leadership were particularly of importance, i.e., "getting on the balcony" and "regulating distress." The study may assist PDS partners and Educational Preparation Providers to augment their existing programs and initiatives by seeking to enhance the mindset that developing student interns is truly an "adaptive challenge" that must be embraced by all professionals surrounding the developing teachers.