DS will turn 5 in early fall (his FSIQ on WPPSI IV is 99%, GAI 99.9%). He is supposed to start K in the fall at a private school.
We are very tempted to keep him out of K and homeschool him for the year. But don't we run the risk that we will not find a private school that will take him into first grade the year after without K?
He is past the (apparently strict) age cutoff for public schools, and even some privates. We don't really want to homeschool in the long run.

Maybe administer a "standardized test" at the end of your homeschool year (ITBS, ERB etc) and use the results to "prove" to the private school that he has indeed done his K year at homeschool?

Perhaps a bigger concern is whether they will have many spaces for incoming students at the 1st grade level, if they fill the cohort at the K level.

You are right! In my area, the sought after private gifted schools are filled at K and the next opportunity to get in is in 3rd grade. So, that would be a factor to consider.

Is it expensive?
Is it hard to get to?
Will the teachers recognize his abilities and provide appropriate challenge?

Any reason for homeschooling just for the K year?

Are tuition payments eligible for prorated refund if you decide mid-year that homeschooling would be a better fit?

Could you tell us a bit about your thought process behind considering HS?

Thanks for the quick feedback. The arguments in favor of homeschooling are a bit involved: We'd have to move closer to the school (and further from work), and, more importantly, we just found out that the teacher will be one who really didn't click with DS during pre-K (we ended up taking him out; so she probably does not like us very much). Essentially, she just ignored him (we didn't see him featured in any of the videos or tons of photos they sent out) and didn't push him (easy to do as he is quiet and just blends in). He actually regressed. This makes us think she would do the same this year and not group him with the better (and much older) kids - they ability group at least fore reading/writing. The IQ test results must have come as a surprise to the teacher (the school admits from the top 15% of kids). Breaks my heart to think he'd spend a good part of the day with someone who doesn't like him. He didn't notice last year, as there was lots of play with friends - but K is a full day and fairly academic program. For us, the biggest worry is whether a year of homeschooling would make it impossible to get into 1st grade the year after. Academically, we are sure we can do it (probably better than any school at this stage); it's more a matter of signaling and social/behavorial concerns (private) schools might have, especially given his late birthday.

Oh, and no, tuition is not prorated.

Have you considered that in the single year your son may cover more than the K material and slotting him into school thereafter may be a problem from that angle too?

Of course, that is not to say that he won't do K and still have the same issue AT school...

I'm not sure what you should do, we are a homeschooling family and DS (7) admitted loving that "my teacher will always love me".

Most people here have had to consider Homeschooling even as a temporary option at one time or another, so you would not be alone and I am sure that as long as you could maintain a portfolio to show that he ticks all the school readiness (1st grade) boxes thereafter they won't say no to him.

rac, after reading your follow-up comments, I can understand how you would be apprehensive about committing to a year of studies with that teacher. I likely would be.

Can I ask some more questions to help you structure the situation?

Assessing probability of securing a grade 1 (or later) space:

-Do you have a good sense of intake levels for the school for the early elementary years?
-Do you know who the usual teachers for the next few grade levels are and whether they'd be a better match for your son?
-Is there a possibility of deferring your son's enrollment until next year?

School awareness of your son's needs & relationships

-What does the principal of the school think of your son's test results?
-Do you have a relationship that would allow you to express some of these thoughts informally?
-Is the school open to skipping or accelerated placement in the likely event that you cover beyond K curriculum?

Here's a thought related to that-- one possible/plausible advantage to homeschooling (or maybe it's a disadvantage, actually)-- is that when homeschooling a child through PreK~4, you have to be pretty conservative in order to not gain on academic readiness at about a 2- or even 3-to-1 rate.

So if you homeschool K for a year, you might actually wind up needing a placement at 2nd or even 3rd grade the year following.

Here's why I say that could be either a good or a bad thing: 1) if the school is WILLING to trial entry at a higher-than-chronological placement, you can get a "freebie" grade-skip out of this, and one that matters the most, since those early grades tend to be where the absolutely worst fit occurs, but 2) if they WILL NOT entertain the notion of an out-of-age placement upon entry, it could well make the fit far, far worse.

We lucked out and got the former-- after a "year" (okay, it was 18 mo or so) of "PreK and K" our virtual charter school didn't cavil a bit over a 3rd grade placement (we provided them with a home-proctored CAT-5 at second grade, which DD had handily 99'ed). If they had balked, however, we just would have said "thanks, but no thanks," and figured something else out. It was pretty clear that a 1st grade placement would have been utterly wrong.

Just something to think about. Plan your homeschooling activities accordingly.

HK said what I was trying to say Just much more eloquently and to the point.

Thanks for these thoughtful questions. One complication is that we may be looking for jobs, which may relocate us to another state, if we are lucky. How I curse that silly age-cutoff.
If we were to stay, I think he would get into that particular school the year after in 1st grade (even without K this year), though I would think they would want to reevaluate him in the spring. I don't believe he would be ready for 2nd grade in that particular school the year after, mostly because it appears to be fairly writing intensive. And they do appear to differentiate in reading/writing. Math is an entirely different question - I doubt he will be challenged.
At most other schools, I am quite sure he could enter 1st grade right now - and still be ahead in math and probably reading (looking at a standard 1st grade curriculum). And this is with *very* little formal instruction (and with English being his weakest language). And yes, HowlerKarma, we realize that if we formally homeschool, it may actually increase the gap with 1-1 attention - he learns so quickly; we could probably cover a few years in math in particular. So far, we have on purpose focused on stimulating his mind in ways that are not necessarily academic, but that he enjoys - 200 piece puzzles (that are tedious to us), advanced Lego, learning about space, earth, the body etc. And on teaching reading and writing in one of his native languages. He also spends hours a day drawing and playing with his younger brother - in that sense he is a perfectly normal kid (and full day academic K may be quite long).
We don't really feel comfortable talking to the school about our concerns, as they have only that one class for K, so there is not much they could do. We don't know why the teacher and DS did not click. But us complaining and then taking him out after a few months certainly doesn't help matters. We don't know how good of a match the next teacher would be.