Most posters that agree with mandatory education seem to be operating under the notion that somehow children will, en masse, run away from education to never been seen again. I think it's fairly ridiculous. This view essentially treats children as second class citizens, without a mind of their own, justifying coercion for their own "good". I don't believe much would change if education becomes voluntary.
Children don't have full rights, nor should they. Children are by nature at various stages of development. Some may be able to grasp the consequences of skipping school, others may not. And some may know and just not care (I've been there). It may be that with a voluntary system, most parents would insist on their kids going to school or else. But it would be a mistake to assume that 5-12 year olds (and even some teens) are generally capable of making such major life decisions.
That is not to say that I think there should be a one size fits all, 100% public approach to education. Far from it, I view the public school system as in need of serious reforms. I would prefer a voucher/charter centered system with the option of apprenticeship or (in rare cases) home school for some. And I'm totally opposed to the Department of Education, No Child Left Behind, etc. But there needs to be some sort of safety net in place to ensure that children are learning
something.