Just out of curiosity -- I have no settled view on the matter -- why would it be so unhappy for "mandatory universal pre-school," a.k.a "school," to start at, say, age 4 rather than age 5 or 6?  Is there some reason to think that kindergarten or first grade is just the right time to start school?  Is your point just the political one that advocating such early schooling would play into the hands of the contemporary right's opposition to making voluntary pre-school more widely available?  Or is there a substantive reason to prefer voluntary to mandatory school up to a certain age and thereafter have mandatory school?

"Happily, no such plot actually exists. Less happily, it very much does exist in the fevered mind of the contemporary right..."

And you said the G.O.P. was short on ideas!

Isn't Rob Reiner proposal in California to do exactly that? Preschools would have to have credentialed teachers, teach state standards, etc. The state would spend just as much per-student on preschoolers as everybody else. It's not a very good deal for the 60% or 70% of California families who already send their kids to perfectly decent private preschools.

Mandatory pre-school strikes me as a bad idea. It's true that 4 years doesn't seem any more arbitrary than 6 years, but you have to draw the line somewhere. My guess is that the age for schooling is determined by the point at which we expect to teach academic skills, most obviously literacy and arithmetic. Some kids are reading at 4, but many are not, and I think most people consider that acceptable.

Life skills and art activity can probably be handled at home. I think preschool is very beneficial, particular just by having kids socializing out of the family from early on. However, I think that the parental right to autonomy far outweighs any benefits at the preschool level.

How mandatory is K-12 schooling anyway? Home schooling is legal, though the degree of regulation varies by state. I would not favor any system that made it impossible for parents to opt out of the standard education system, provided their kids met reasonable academic goals.

 I don't agree with a number of specific points you make here, but the gist made me see something more clearly.  My point was not that 4 is no less arbitrary than 6, but rather than 4 may very well be better than 6.  I'm not sure I recognize "the parental right to autonomy" as regards their children, as distinct from themselves.  In fact, the (future) autonomy of children is one of the main justifications of early childhood development (along with other justifications, like the role of ECD in economic development and addressing the challenges of contemporary parenting).  And you're right that K-12 is not mandatory, strictly speaking, though it or some equivalent is, which is close enough.
This last point made me see what may be implicit in Matt's post: make high-quality voluntary preschool more widely available and, given the costs of child care and the wide spread of working parents, most parents these days would be crazy not to get their children involved.  Then you would end up with a lot of the benefits of mandatory universal preschool without the political controversy.  A lot depends on what "available" means here, in terms of cost, as well as how widely you make it available.  It might be too that a lot of the children who would benefit most from some robust ECD program have parents least likely to be responsive to voluntary incentives, although there might be ways of addressing that.  The Minneapolis Fed has been doing a bunch of work in this area. See http://minneapolisfed.org/research/studies/earlychild/ 

Thanks for the new reply.  I agree there are a lot of ways to go short of mandating preschool, though the one you suggest strikes me as worse than others I can imagine, which would try to prevent rather than redress a two-tier system among students from the very getgo.
There may be some ideological disagreement between us, though I see mostly pragmatic and empirical issues.  Like you, I'm skeptical of the ability of anyone to render authoritative decisions on parenting, while seeing the need for them.  And I do think that a law requiring public education or some equivalent fully justifiable, which opens up the mostly empirical question of when such education is best begun.  I assume, though maybe I shouldn't, that you don't see public schooling per se as an unjustifiable infringement on the traditional and biological role of parents.

I assume, though maybe I shouldn't, that you don't see public schooling per se as an unjustifiable infringement on the traditional and biological role of parents.

I think I answered this in the first posting. The law already provides an opt out in the form of home schooling. I can't say I have ever given much thought to it but it seems like a reasonable balance. I probably disagree with many home school parents on their reasons why, but I think it is good public policy not to prohibit it.

Why not provide free pre-school, nevertheless, for those other 30 - 40% who simply can't afford it.  If there is evidence showing that pre-school can be beneficial, and I think that's not a disagreement here, why not at least offer it to the many low income families in this country?

I lived in Georgia until 2001 where preschool was available on the public's dime, courtesy the lottery (don't know if this is still the case).  At the time, both public schools and private day care centers like the YWCA operated thre pre-K programs.  It was wildly popular.  In Athens, Georgia, I stood in line for several hours with other parents in hopes of getting our kids in the public school Pre-K program.  I also spent the night outside the local YWCA with other parents in hopes of getting our children a place in that program.  And then I moved to Alabama where there is little to no pre-K and we had to spend our own money (something like $150 a week) to send our daughter to a private pre-K program.  We could have spent less, as little as $85 a week if we had been willing to send her to a place staffed by minimum wage employees and no real curriculumn.  I have spent lots of time investigating pre-K programs in my area and most of them are horrible.  The only exceptions are the high priced montessori-type schools.  People who are opposed to the government voluntarily funding pre-K programs have no concern for the real life struggles of two-income families.   For us, having my wife or me stay home was simply not an option.  Bring on the government funded pre-K I say.  The home-school option would seem to mute most protests even if you assume the most slippery slope process possible.