European societies and schools face the challenge of accommodating immigrant minorities from increasingly diverse cultural backgrounds. In view of significant belonging and achievement gaps between minority and majority groups in school, we examine which diversity approaches are communicated by actual school policies and which approaches predict smaller ethnic gaps in student outcomes over time. To derive diversity approaches, we content-analyzed diversity policies from (n = 66) randomly sampled Belgian middle schools. Cluster analysis yielded different approaches valuing, ignoring, or rejecting cultural diversity in line with multiculturalism, colorblindness, and assimilationism, respectively. We estimated multilevel path models that longitudinally related diversity approaches to (N = 1,747) minority and (N = 1,384) majority students’ school belonging and achievement (self-reported grades) 1 year later. Multiculturalism predicted smaller belonging and achievement gaps over time; colorblindness and assimilationism were related to wider achievement and belonging gaps, respectively. Longitudinal effects of colorblindness on achievement were mediated by (less) prior school belonging.