The objective was to characterize the relation between different sources of school-based social support (friends, peers, and teachers) and bereaved siblings' grief and grief-related growth and to examine whether nonparental sources of social support buffer the effects of low parent support on bereaved siblings. Families (N = 85) were recruited from cancer registries at 3 pediatric institutions 3-12 months after a child's death. Bereaved siblings were 8-18 years old (M = 12.39, SD = 2.65) and majority female (58%) and White (74%). During home visits, siblings reported their perceptions of social support from parental and nonparental sources using the Social Support Scale for Children, as well as grief and grief-related growth using the Hogan Sibling Inventory of Bereavement. Parent, friend, and teacher support were positively correlated with grief-related growth, whereas parent and peer support were negatively correlated with grief for adolescents. Teacher and friend support significantly moderated the association between parent support and grief such that teacher and friend support accentuated the positive effects of parent support. Friend and peer support moderated associations between parent support and grief/growth for adolescents but not children. School-based social support, namely from friends, peers, and teachers, appears to facilitate the adjustment of bereaved siblings. Findings suggest that bereaved siblings may benefit from enhanced support from teachers and friends regardless of age, with middle/high school students particularly benefitting from increased support from close friends and peers.