Purpose: The authors developed online learning modules to train graduate and undergraduate student instructors (GUSIs) on grading and delivering feedback in quantitative disciplines. The authors report results from multiple assessments conducted during recent training events at a mid-sized, research-intensive institution and discuss implications for educational development. Design/methodology/approach: Using pre/post-assessments, the author measured participants' learning gains and skill development. In Study 1, the authors measured learning gains for 109 computer science GUSIs randomly assigned to complete the modules or not. Participants who completed the modules performed significantly better on the post-assessment relative to the control group across all seven module learning objectives aligned with GUSI responsibilities. In Study 2, we iterated on both assessments and modules, replicating Study 1 results for GUSIs from other quantitative disciplines. In Study 3, the authors compared learning gains from online modules to in-person training sessions, focusing on the authentic task of providing written feedback on student work. Findings: Proficiency improved equally and significantly via both training modalities. Originality/value: At research-intensive universities, GUSI training can be inconsistent and difficult to scale and rarely assessed via direct measures of outcomes. To the authors' knowledge, this is the first study to rigorously measure GUSI skill development via authentic assessment tasks such as grading student work and/or providing effective written feedback rather than simply testing knowledge. This study also addresses implications for designing and implementing effective GUSI training at scale.