Although schools with a high concentration of ethnic minorities often underperform, not much is known about the factors that predict high and even excellent achievement of pupils within these schools. Therefore, we examined which concrete diversity practices and key psychological processes increase or decrease the likelihood/odds for high and excellent performance on a standardized mathematics test, thereby utilizing multivariable logistic regressions among 855 10-12-year-olds in 18 majority-minority schools in Flanders, Belgium. While the odds for high and excellent mathematics scores were unaffected by language spoken at home or religiosity, they were negatively related to deficit-based assimilationist school practices, inconsistently related to meritocratic neutrality-based practices, and positively related to a strengths-based anti-racist school curriculum. Furthermore, while higher school belonging and a growth mindset increased the odds of high and excellent achievement, students' experiences of peer or teacher discrimination decreased these odds dramatically. Implications for school effectiveness policies are discussed.