An educated teacher would say, if the issue of the morality of slavery came up, that in today's world, slavery is universally condemned and is outlawed in most of the world. However, there are still about 30 million slaves held across north Africa and in the Muslim world, which while ostensibly prohibiting it, allows it in practice. In the past, however, through most of human history, slavery was a legal and accepted concept and practice.
If a high school teacher was going to get into the morality of slavery, a great way to do so would be to have the kids study the reasons why slavery was accepted in past societies, and why a practice so universally condemned today was once believed to be normal and moral - some societies thought slavery to be god-commanded.
Morality depends on one's culture, one's religion, one's philosophy, etc. And, there were full expositions attempting to defend the institution of slavery in the past. These are quite interesting to read. A major writer in this area was George Fitzhugh. See -
https://docsouth.unc.edu/southlit/fitzhughsoc/fitzhugh.html and
https://www.gutenberg.org/files/35481/35481-h/35481-h.htm Many other writers are out there - Thomas Drew, for example. Aristotle wrote about the rightness of slavery -
https://www.jstor.org/stable/640902
Remember, it is the mark of an educated mind that a person would be able to entertain an idea without accepting it, and a history teacher is not there to tell us what to think in a moral sense, but to arm us with the historical facts on which to conduct analyses and define morality and/or make value judgments.
Is it a history teacher's job to tell us that an assassination is "bad" (when studying, say, the start of World War One)? Or is it the teacher's job to teach history, and if value judgments are discussed, to study the value judgments, not simply declare one side to be the moral one? Was the assassination of Franz Ferdinand justified? Serbia put up a statue of Gavrilo Princip to honor him, because he was fighting the oppressive Austro-Hungarian Empire in favor of the freedom of the peoples of Serbia, Bosnia and Herzegovna and to allow for the formation of a southern slavic state free of Austro-Hungarian domination. "The shots fired 100 years ago by Gavrilo Princip were not fired at Europe, they were shots for freedom, marking the start of the Serbs' fight for liberation from foreign occupiers." — Milorad Dodik, a Bosnian Serb politician and president of Republika Sprska.
- interesting, eh? A man put a bullet in a prince and princess in 1914, and we don't have uniformity of opinion as to what moral judgment to make of it. Some folks have a statue honoring the killer. Others view him as a terrorist.