Addressing the impact of peer bystanders on school bullying, this cross-sectional study examined whether student responses to bullying that they witnessed varied as a function of sex and grade. In a school-based survey regarding social experiences at school, Grade 4 to 11 students (N = 9397, 51% male) who reported witnessing bullying (68%) rated how often they had engaged in different bystander responses. Results indicated significant differences across sex and grade level, such that younger students and girls were more likely to report taking positive action than were older students and boys by directly intervening, helping the victim, or talking to an adult. Generally, boys and girls were equally likely to report that they ignored or avoided the person(s) who bullied although reports that they did nothing increased with grade level. Implications for schoolwide antibullying intervention efforts are discussed.