Background: Concussion education is recommended to increase concussion knowledge, beliefs, and reporting intentions. The Concussion Awareness Training Tool for Youth (CATT-Youth) is a 40-minute e-Learning module developed for high school-aged youth. Aim: The aim of the study was to evaluate changes in concussion knowledge, beliefs, and reporting intentions in high school youth from Calgary, Canada, following completion of the CATT-Youth. Methods: This study used a modified stepped-wedge trial design. High school classes were randomly assigned to an intervention (Ix) or delayed intervention (DIx) group. Ix group participants completed a pre-CATT survey immediately followed by the CATT-Youth, then a post-CATT survey 2 to 6 weeks later. DIx group participants completed two pre-CATT surveys 2 to 6 weeks apart, with the CATT-Youth completed immediately following the second pre-CATT, then a post-CATT survey 2 to 6 weeks later. The pre-/post-CATT survey encompassed 11 subtests evaluating concussion knowledge, beliefs, and reporting intentions. Independent mixed linear regression models were conducted to examine changes in scores for each subtest. Results: Participants included 454 high school students: five Ix schools (16 classes, n = 323) and two DIx schools (six classes, n = 131). The CATT-Youth significantly increased general concussion knowledge, Ix [delta] = 0.546/8 (95% confidence interval [CI] = [0.243, 0.849]), DIx [delta] = 0.728/8 (95% CI = [0.389, 1.106]), and beliefs about capabilities, Ix [delta] = 2.462/28 (95% CI = [1.086, 3.838]), DIx [delta] = 3.219/28 (95% CI = [1.594, 4.844]) for both groups. For some subtests, improvements were noted in the DIx group only. Conclusion: The CATT-Youth module improved concussion knowledge and beliefs about capabilities for students in both groups. Future studies should explore the utility of the CATT-Youth in changing knowledge, beliefs, and reporting intentions in high school students.