I think it should be up to the children - with guidance from their teachers - to decide whether they should be applying 20th century ethics to 15th century events.
That's what education is really about.
I agree with you - but personally, I still can't find it in me to think that slavery was ever considered to be an acceptable practice by the majority of the populace in any era. I think what makes the difference is that power structures have changed, and paradigms have shifted that now allow individuals more control over their destiny.
Serfs who toiled for lord and manor were essentially slaves tied to the land and eked out lives that sound pretty miserable for hundreds of years - does that mean that they were OK with it? In the 1300's European populations were decimated by plague, and things changed, and labor was suddenly in demand. Good-bye serfdom!
I just see it as they had no choice because of the situation they were in - that doesn't mean that the majority of the population thought slavery was OK or an accepted "norm", it just meant the people holding the power did, and those who objected had limited means to change things.
Am I crazy to think this way?