Why? Were you privy to the whole lesson, or just assumed that the teacher showed the film as reflecting historical truth?
He/She might have shown the film to point out the historical inaccuracies and America's fascination with conspiracies.
I showed parts of Birth of a Nation to my class, not because I think the film reflects any historical accuracy, but explained that it's a snapshot of dominant attitutudes about race relations in 1915.