Via @dkavadia I found this article by Germ Janmaat about his research in which he examined the impact of classes with an ethnically mixed intake. He found that ethnic diversity in the classroom does lead to more tolerance of immigrants among teenagers. But the proportion of first-generation and second-generation immigrants makes a difference to attitudes.
From the article:
“These findings suggest that education policies should consider ways to increase ethnic diversity in schools as a strategy to foster inclusive attitudes among the native majority. But we should not expect immediate results in schools with many first-generation immigrant students. Neither should we expect a minimal presence of immigrant students in class to do the job.”