Continued migration flows are inevitable, and research into favorable conditions for educating students in pluralistic societies is timely. This study attempts to address the operationalization of favorable conditions when educating children in diverse societies. Previous research into the contact hypothesis and acculturation theory has suggested that moderating factors are necessary for contact between groups of "others" to effect positive change in attitudes of tolerance and reduce prejudice. To represent groups of "others", this study--which analyzed ICCS data from both the 2009 and 2016 cycles--considers lower-secondary students with an immigration background, who speak a different language than the administered assessment in the home, and diversity in terms of socio-economic background. To represent a desirable or "successful" classroom context, the study focuses on attitudes of tolerance held by lower-secondary students towards minorities, investigating whether participation in civic activities in the school or community, or the act of learning in more diverse classrooms, was associated with increased rates of tolerance. Findings supported the stated hypotheses to some extent, as some associations between heterogenous classroom composition (in terms of the share of students from immigrant backgrounds and those who spoke different languages in the home) and increased attitudes of tolerance towards minorities were observed. Student participation in civic activities in school was significantly associated with higher attitudes of tolerance towards minorities in 16 out of 18 analyzed countries. A relation between civic participation in the community and tolerance towards minorities was not found. Overall, students' rates of tolerance towards minorities increased significantly for the majority of analyzed countries between 2009 and 2016.