This study examines the prevalence of bias-based bullying and the associations among bias-based bullying, negative effects, school avoidance, and supportive adults. Frequency analysis and moderated mediation analysis were conducted using a national sample of adolescents aged 12 to 18 in the U.S. from the 2015, 2017, and 2019 National Crime Victimization Survey School Crime Supplement. Among the adolescents who were bullied between 2015 and 2019 (N = 629 in 2015, N = 1,179 in 2017, and N = 1,197 in 2019), 40.3%-45.0% of them thought that bullying was related to their race, religion, ethnic origin, disability, gender, sexual orientation, or physical appearance. Among the adolescents who were bullied, the most frequently perceived reason was physical appearance (28.1%-30.8%), followed by race (10.2%-11.8%). The presence of supportive adults at school significantly moderates the mediational pathway between experiencing multiple forms of bias-based bullying and school avoidance through the negative effects of bullying.