Treatment integrity of school-based interventions is discussed within the dropout prevention and the Check & Connect (C&C) mentoring program framework. Treatment integrity factors and increasing and measuring it are explored. The psychometric properties of the C&C Survey, an online administered self-report treatment integrity measure, used 2010/2011 data from 24 mentors and middle school mentees. Estimation of the C&C Survey's criterion and content validity through mixed methods included data across multiple factors of treatment integrity and mentee outcomes. Aspects of the C&C Survey correlate to improved mentee attendance and, less so, to decreased mentee GPA. Qualitative data showed mentors with higher C&C Survey scores have more comprehensive knowledge of C&C interventions. Experienced mentors found the C&C Survey accurately reflected their activities. The C&C Survey was deemed to have adequate psychometric properties. The frequent use of online administered treatment integrity measures for school-based interventions is proposed and further investigation posited.