On average across OECD countries, and after accounting for the socio-economic profile of schools, no association can be identified between the percentage of immigrant students attending a school and the likelihood that students attending that school will attain baseline levels of academic proficiency. After accounting for schools’ socio-economic profile, a high concentration of immigrant students was associated with poorer performance in Germany, Slovenia, Spain and Switzerland. Slovenia was the only country where students who attend schools where more than one in four students has an immigrant background are less likely to attain baseline levels of proficiency, after accounting for the school’s socio-economic profile.