Is this "ethnicity" class designed to teach students what it feels like to be different? That's my read. And if that's the case, then it did exactly what it's designed to do. It doesn't mean the teacher was actually ticked off because the kid is a Denver fan.
Remember the age old "test" of brown-eyed students vs blue-eyed students, which was designed to teach students what it feels like to be the subject of discrimination?