Currently there is a call for brief concise measurements to appraise relevant 21st century college readiness skills in K-12 learners. This study employed K-12 teachers' ratings for over 3,000 students for an existing 91-item rating scale, the Human Behavior Rating Scale, that measured the 21st century skills of persistence, curiosity, externalizing affect, internalizing affect, and cognition. Teachers' ratings for K-12 learners were used to develop a brief, concise, and manageable 30-item tool, the Human Behavior Rating Scale-Brief. Results yielded high internal consistency coefficients and inter-item correlations. The items were not biased with regard to student sex or race, and were supported through confirmatory factor analyses. In addition, when teachers' ratings were compared with students' academic and behavioral performance data, moderate to strong relationships were revealed. This study provided an essential first step in the development of a psychometrically sound, manageable, and brief tool to appraise 21st century skills in K-12 learners.