Okay, good answer.
But the problem with this (according to generalizations I can draw and a few observations I've made) is that it caters to those who I would call for want of a better word, losers. Slackers. Kids who don't give a shit. Just imagining my average high school acquaintance -- a few would positively thrive, and the rest would crater. It encourages a lack of order in life, and as you point out, the rest of the world is about order. Going to college from a Montessori would be like moving from a Greek island to Manhattan.
In my opinion, that's why this sort of educational system will never catch on widely. I firmly believe children should attend public school for the social immersion, though, so I'm on the extreme end of this question's spectrum.