Previous studies have shown that schools' socioeconomic-status (SES) composition has an impact on the academic performance of pupils. Less attention has been given to the explanation of this effect. This study examined whether the teachability culture among the school staff (teachers' collective beliefs about how teachable their pupils are) mediated the school SES effect on science achievement and achievement growth. Multilevel analyses were conducted with data from 1,761 pupils and 1,255 teachers across 66 primary schools in Flanders. First, the analyses indicated that there was a positive association between school SES composition and teachability culture: Even after controlling for cognitive ability and performance of pupils, there was a more pessimist culture in socioeconomically disadvantaged schools. Second, the association between school SES and academic performance was explained/mediated by the teachability culture. However, no school effects or mediation effects were found for achievement growth as the covered period of academic growth was too short.