Can someone please help me write an apology letter? Please?

I don't really why i am apologizing because what happened was that i was job shadowing this nurse for a class at a hospital, and they were talking about native americans and their features and they were talking about how this nurse went to this restaurant and how this "native american" woman was mean to her and the nurse was talking about her facial features and how bad the native american person looked but she kept referring to them as "indians" i am from the country india and i asked her politely to quit talking about these native americans or quit refereeing to them as indians ..and thats all i said but the next day they called my instructor and had me removed for offending the nurse's intelligence and now i must write an apology letter to pass the class so could someone please help me write this letter i don't know what or where to begin this letter. best answer gets 10 points :)