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.
[1] http://en.wikipedia.org/wiki/Slavery