Abstract: This study proposes a tensor-based instructional framework that formulates Schmid’s law for single crystals and extends it to the description of polycrystalline equivalent yielding. At the single-crystal level, the resolved shear stress is expressed as the double contraction of the stress tensor and the Schmid tensor, enabling a unified tensor formulation to determine the activation of slip systems under various loading conditions. At the polycrystalline level, based on the Taylor uniform strain assumption and the minimum plastic work principle, a primal–dual pair of optimization problems is constructed to determine the yield-point stress tensor and characterize the yield surface. This framework reveals the intrinsic connection between microscopic slip mechanisms and macroscopic yielding conditions, providing theoretical support for constructing a coherent “micro–macro” knowledge structure in mechanics education.