Employers agree that high school graduates lack teamwork skills needed for workplace productivity. The lag in student readiness to meet employer expectations urges school reforms that implicate group interaction, empirical assessment, and reporting of teamwork competencies. The "Teamwork Skills Inventory" (TSI) evaluated how well individual students performed in cooperative learning teams as observed by peers and self with anonymous feedback. Field-testing involved 303 high school students in the United States from middle class and racially diverse families. Psychometric indicators of reliability, validity, exploratory factor analysis, and group analytics were determined. Qualitative responses by participants to TSI methodology were highly favorable.