Many governments in post-conflict settings attempt to erase their violent pasts, leaving out all or large parts of their history from school curricula. While some hope this policy will help individuals forget the past and move on, it may simply allow local, polarized versions of the past to flourish, fueling current divisions. When past conflicts are not discussed openly in schools, what and where do younger generations learn about their nation's violent past, and how do these narratives affect inter-group relations? When are resentments and heightened prejudices sustained and passed down to youth, and when do they fade? I answer these questions using evidence from Nigeria - which has rarely taught its history in the 50 years since its Biafran Civil War. Surveying 2584 secondary school students and their parents across 1292 households, I find that without formal education most families have passed on surprisingly few details about the war to their children. Children are also unlikely to learn much from other sources. However, the (vague) knowledge of the war youth do have tends to be ethnically charged, particularly among the most victimized group, and this sentiment is correlated with current ethnic prejudice and support for secessionist movements. Crucially, these findings hold both in places with contemporary grievances and segregation, and those with integration. This suggests the passage of time and inter-ethnic contact may not be enough for past tensions to fade, but instead more direct engagement with details about past conflict (including through formal education) could help peace-building.