Various policy papers and research studies assert that teamwork is one of the most important skills for students to learn if they are to become meaningful contributors to the 21st century workforce. However, outside of organizational psychology and adult populations, few reliable assessments of this construct exist, with suitable validity evidence scant or nonexistent. To redress this imbalance, teamwork assessments for high school students were developed using multiple methods: self-report ratings, situational judgment testing, and teacher reports. Exploratory factor, confirmatory factor, and latent class analyses were used to determine the structure of the scales. Measures showed reasonable reliability and promising validity evidence, relating to each other and to academic achievement, while remaining relatively independent from personality. The advantages and disadvantages of each methodology and the potential applications for identification and intervention, selection, and evaluation of training programs are discussed. This report also serves as an archival document for the teamwork and collaboration assessments that have been developed at ETS for high school students.