Spirit Week is coming, and one of the themes is Cowboys vs. Indians. Automatically after hearing that I thought of culture appropriation, imagining people coming to school in headdresses and war paint, is this racist, or am I overreacting?

I understand why you feel uneasy with this. Cultural appropriation is something I still haven't quite got my head around. It's really difficult, but I do think this has racist undertones. You could bring it up with whoever is organising the event to explain your concerns.