I think it would require a fair amount of discipline on the part of the parents. There are always going to be children who would, given the chance, sit around all day doing nothing worthwhile.Not just formal schooling, but also formal homeschooling - i.e. not using a strict curriculum as a lot of homeschoolers do (we currently use a curriculum). It's completely self-directed learning; basically giving the child access to any resources they want and letting their curiosity dictate what they learn.
I've seen cases where it can go quite badly as well, for instance some young people we know are getting to the age of matric/GCSE, and incapable of passing the standard needed, and basically at a point of dropping out of further education because they never learned how to study. That's what has made us more nervous about taking an unschooled approach. On the other hand I believe it can work wonderfully in other cases like in a worldschooled situation where they have so much environmental stimulation and opportunity.
On the other hand in my experience the standard method of schooling typically also fails to teach how to study and, because school level education essentially involves support and spoon feeding all the way to the final year, university is a huge shift that leaves many floundering.
Montessori seems to take a bit of a middle ground. There's guidance and direction to make sure the various areas required are covered, but even four year olds are expected to take some initiative and from an early age they engage in teaching what they know to those younger or less knowledgeable.
