Context Version 5 of the Sport Concussion Assessment Tool (SCAT5) was released in 2017 with an additional 10-word list option in the memory section and additional instructions for completing the symptom scale. Objective To provide reference scores for high school rugby union players on the SCAT5, including immediate memory using the 10-word list, and examine how age, sex, and concussion history affected performance. Design Cross-sectional study. Setting Calgary, Alberta high schools. Patients or Other Participants High school rugby union players (ages 15–18 years) participating in a 2018 season cohort study (n = 380, males = 210, females = 170). Main Outcome Measure(s) Sport Concussion Assessment Tool 5 scores, including total number of symptoms (of 20), symptom severity (of 132), 10-word immediate memory (of 30), delayed memory (of 10), modified Standardized Assessment of Concussion (of 50), and balance examination (of 30). Results The median number of symptoms reported at baseline ranged from 5 to 8 across sex and age stratifications. Median symptom severity was lowest in males with no concussion history (7; range, 0–28) and highest in females with a concussion history (13, range = 0–45). Median total scores on immediate memory were 2–3 (range = 0–4) for males and 21 (range = 9–29) for females. Median total scores were 3 (range = 0–4) on digits backward and 7 (range = 0–20) on delayed memory (all groups). Based on simultaneous quantile (q) regression at 0.50 and 0.75, adjusted for age and concussion history, being female was associated with a higher total symptoms score (q0.75 βfemale = 2.85; 99% confidence interval [CI] = 0.33, 5.37), higher total symptom severity score (q0.75 βfemale = 8.00; 99% CI = 2.83, 13.17), and lower number of errors on the balance examination (q0.75 βfemale = −3.00; 99% CI = −4.85, −1.15). Age and concussion history were not associated with any summary measures. Conclusions The 10-word list option in the memory section reduced the likelihood of a ceiling effect. A player's sex may be an important consideration when interpreting the SCAT5 after concussion.