I read about this not all that long ago.
The funny thing is, that when I was young (and a devout Christian), I struggled with this very issue. I asked other Christians, whom I respected, why the Bible seemed to indicate that the institution of slavery was not a sin. The answers that I got were never very satisfying, but there were a number of answers, and for the sake of discussion, I will introduce them here. Before I do, however, I want to make it clear that I am not a Christian any longer, and I do believe that slavery is without question evil.
One of the most common, albeit weak, Christian defenses of the Biblical stance on slavery went something like this: Slavery was inevitable in the ancient world, and to have prohibited it entirely would have been impractical, so instead, the Bible laid out a series of provisions for how slaves were to be treated, and so on.
While it is true that there are some Biblical restrictions on slavery (including a possible seven year time limit) - it is really more of a dodge than an answer. Slavery is wrong because it is immoral for one man to, by force of arms, deny another man his liberty for no purpose other than to rob him of the product of his work. So arguing that “limited slavery” is somehow ok seems rather weak.
A better (in my opinion) response went something like this: In the Old Testament, the only thing that people had was the law. The law was cruel, and imperfect, because the world is a cruel and imperfect place. The New Testament (because of Jesus) ushered in the concept of “grace” or forgiveness from the law. It is this very concept of grace, and the concept that Christians are free from the strictures of the law that make it acceptable for a Christian to eat pork, for instance.
Unfortunately, this concept of “grace” really muddles any attempt to know for sure if something is right or wrong, with many leading Christian scholars arguing that the deed is not as important as the intent. Unfortunately, as it pertains to slavery, it does not offer a condemnation of it, but only fails to condemn someone who, for themself, would opt not to keep slaves. If the NT had failed to condemn any practice outright, I might be able to write this off as the religious equivalent of avoiding confrontation, but as the NT does clearly go out of its way to condemn some behavior (such as homosexuality) this does not seem to hold up.
Finally, and this was a minority response (but keep in mind that I am a California boy, and even Christians on the left coast tend to be a bit more progressive than many of their counterparts elsewhere) some people expressed the following opinion: Slavery is not wrong, per se, but is the province of government. As Christians, we are to live in accordance with the law of man and the law of God, unless the two are in conflict, in which case God’s law is superior. Thus, in America, slavery, having been outlawed, is illegal, and because it is not foundational to salvation, as Christians we abide by the law of man in this matter.
This was a better response, in my opinion, inasmuch as it did not dodge the fact that the Bible did, in fact, indicate that slavery was ok. It was, however, ultimately unsatisfactory, because it failed to put my mind at ease, inasmuch as I had a sneaking suspicion that slavery was wrong period.
Unfortunately, human nature being what it is, I feel that the lack of clear prohibition against slavery in the Bible, and thus in the shadows of American thought, has left America open to some pretty horrific possibilities. This school case is only one of them.
Consider the deplorable case of our prison labor system. If one looks at it under the right light, it looks an awful lot like slavery.
Consider the fact that farms are allowed to bypass many labor laws, including age limits and minimum wages.
Consider that many “illegal” immigrants work in bad conditions, for inadequate pay, but don’t say anything for fear of getting caught.
Consider the recently passed bankruptcy “reform”. It is now harder for people who screw up with credit cards to go BK, which means that they may be stuck, in effect, working for their creditors for years.
Slavery, as both a concept and an institution, is alive and well in America today. It is evil, but you won’t hear the religious right opposing it anytime soon.