On a different note... I think the key takeaway here has less to do with what a school is, and more to do with how it relates to authority and autonomy.
I'm sympathetic to his view of school as a "magic womb" away from the reality of everyday existence. And yes, that can sometimes be a bad thing, but not always.
I'll take Pippa's lead, and personalize this. I got kicked out of a private school early in the eighth grade for non-payment. This was after getting kicked out of other schools for that or for any number of other things. Our family went through a lot of financial swings. The school had kept me on longer than they ought to have because I and my sibs helped their showing on standardized tests. But I had a problem with authority--or more exactly authority had a problem with me. I didn't fit.
Soon after I was... well, "home-schooled" is probably the wrong word, and "unschooled" suggests that there was some thought involved in the process. When I was 14 my mother encouraged me to return to school, and given that we were homeless at the time, I picked the best public high school in the region. Why not?
It was a disaster. I didn't make the week. I jumped in in the middle of the semester. New students were given a "buddy" to help them adjust. Mine suggested paying up to the local bully and going shopping for clothes that would make me stand out less--neither of which were possibilities. I was accused of cheating in the first couple of days because my homework was "too good." (And because I had accidentally ended up in two senior level classes and still found the work facile.) I literally was spat on by other students twice during my first day, and figuratively spat on by the teachers. It was about as hellish an experience as I could imagine, and so I walked out at lunch of my fifth day, and never looked back.
Now, I really could have used a womb. A place apart from my everyday life, which involved caring for my siblings, staying a step ahead of the law, and trying to figure out where food was going to come from. This isn't a pity party--I had a fine childhood, and there are others far worse off. When I see kids--especially girls--in places like Afghanistan (as well as closer to home) who are provided with schooling, it is really hard for me to think "stop treating this young people like children, get them out into society." Because, frankly, sometimes society sucks more than schools.
I think the ideal school is a kind of utopia (even if schools play a remarkably minimal role in More's Utopia), a place where learning can occur without the distractions of everyday survival. I think that ideal of school is found in the Harry Potter books. Yes, there is trepidation, but for Harry, it's a space where he can become more autonomous and adult-like. Though there are hierarchies, they are largely of merit (though I suppose hereditary as well) and not of age. So he has to play the child in the muggle world only to escape it at school.
I think Illich misses this somewhat. Yes, the social construct of childhood tends to be extended much longer in the wealthiest countries, but for many, schools represent the way out of backward, structured, traditional societies in which the only way to exercise power is to grow old. The idea that age is directly related to role, task, or power is not unique to the school. And the best schools can provide a respite from such hierarchies and prejudices.