The aim was to investigate the effects of school mobility on achievement in compulsory school in Sweden for children in out-of-home care (OHC) and not in OHC (NOHC). Register data on background variables from four birth cohorts in the cohort-sequential longitudinal project ETF was relied upon, along with student performance on a test of cognitive ability, and school grades according to the leaving certificate. Yearly data concerning school mobility also was available. Results showed that relocation was associated with lower grades for both the OHC and the NOHC groups. The mean effect of one school relocation was 5 percentiles lower grades, but the effect of relocation was twice as large at the end of compulsory school. Given a higher frequency of relocation for children in OHC, they were more negatively affected. School mobility thus is one of several factors which contribute to the poor school achievement of children in OHC.