In a world where there are an estimated 12 to 27 million actual slaves[1], none of whom are free to leave their servitude under penalty of bodily harm or death, I don't think it would be fair whatsoever to compare school (even compulsory schooling of children) with slavery. Slavery is a completely brutal and unacceptable institution, where one party has absolutely no rights.
This is like the inevitable comparison between the US government stealing our freedoms and China (or some other dictatorship). The argument isn't that they are exactly the same thing - it is that one is edging closer to the other than we would like, and we would like to draw attention to the bad similarities in an attempt to fix them.
If we never compare the way things are to the worst ways we know, how can we know if we're falling down that slippery slope until we get to the end and it's too late? Maybe upon reflection we'll realize that we're already several steps down a bad path, even if we're still far from "evil" territory.
[1] http://en.wikipedia.org/wiki/Slavery