The form and function of a collaborative assessment known as a "Buddy Quiz" is presented. The assessment is conducted in three successive phases over a contiguous 45-60 min class period. A portion of each quiz is completed in collaboration with one or two peers and a portion is completed without collaboration. The quiz is primarily summative and is also designed to include formative aspects. The representation in the quiz of the scientific enterprise as collaborative and individualistic is discussed. The employment of this instrument in a ninth-grade (age 15 years) conceptual physics course in an independent US secondary school is described and student feedback is presented. (Contains 2 figures, 2 footnotes, and 1 table.)