What a great question. Let me suggest a few ways.
First, you are actually in an ideal place to talk about tolerance with respect to all cultures, because you are in a position to educate. Since people learn from not only sitting under a teacher, but also by watching and observing, you will need to model tolerance. This is essential.
Second, since much intolerance is rooted in ignorance, you can teach several units on some of the great figures in history from around the world. In this way, students will begin to see people as people, who are just like them.
Third, you can talk about important social frameworks that challenge any one dominant worldview. What is commonsense in one place is not commonsense in another. By doing this, you can give a firm intellectual foundation on the practice of tolerance. Also you can emphasize the importance of diversity.
Fourth, you can have every students share about their backgrounds. The great part of doing something like this is that all people in America are immigrants, unless you are a native American. This exercise will show that tolerance was extended to them in some point in their family's life.