Intervention researchers in school settings often implement interventions that involve professional development to schoolteachers or other professional staff to implement school-based interventions. In terms of classroom interventions, teachers are the primary implementation agents; therefore, the fidelity of such interventions depends on teacher adoption and delivery. To understand and meet such fidelity challenges, we set to identify the contextual components and implementation mechanisms that contribute to the variation in teachers' perspectives and methods of implementing an experimental evaluation of CareerStart; an innovative intervention designed to increase student school engagement and achievement by advancing teacher use of career relevant instruction in the middle grades. The results of this study suggest teachers perceive CareerStart in three interrelated ways: their perceptions of the program's effectiveness on student outcomes; the degree to which the program is adaptable and compatible to their roles and responsibilities; and the effects of the school environment on their perceptions of the intervention. We conclude with a discussion of treatment fidelity effectiveness in school-based intervention programs.