One of the justifications I hear for avoiding certain city schools for Kindergarten has nothing to do with bad teachers, bad buildings, low fundraising or behavior: it's the "giftedness" issue.
It scans something like this: a lot of the children at School Y don't know the alphabet when they start, so my child, who does, will be bored and unchallenged.
I am not terribly sympathetic to this argument. In general, my students don't know the alphabet when they start. Learning it takes around five to ten minutes of our school day for whole group instruction, doing activities that are fun for everyone and teach more skills than letter names (for instance, activities that hone eye-tracking or hand-eye coordination, or patterns, or teach cooperative skills, or teach classroom structures that we all need to know - since alphabet teaching doesn't last all year, and I need to teach these structures, it's an important and valid educational goal). Everything else is in small group, targeted to the needs of the learners. After all, it's not really "knows the alphabet" and "doesn't" - there are the kids who have sorted everything but b d p q, the kids who know the capitals but need work on lowercase, the kids who know eighteen letters, the kids who need to sort out the category "number" and "letter".
And again, I'm not doing this all year. And not for a long time each day: I take teaching science and art seriously, we have other aspects of reading than alphabet recognition to cover, and so on.
My other problem with this argument is personal. I was a very gifted (if also very hyper) student. I was reading before I started Kindergarten and not just a little, either. By the beginning of first grade I'd finished off the Ramona Quimby books and starting in on Joan Aiken.
In Kindergarten, I was absolved from penmanship practice and phonics workbooks (about fifteen minutes of the day) in favor of additional recess with a couple of other early readers. In first grade, the reading teacher pulled a high group from the class, and I went for awhile, but I was too high for that group too, so I stayed with my class.
I was also high in math, and honestly? I wasn't bored very much. This may be thanks to the magic of ADHD, Saving Children from Boredom By Providing All Kinds of Bad Ideas. It wasn't due to excellent differentiation in first grade, either: I did the same work as everyone else. But the work we did didn't just teach reading: it also taught skills that kids need. Some of these - like paperwork skills - I really wish we didn't need. But we do. They also included games, and games are fun even if they're easy.
Nor do I think I could have gone further faster had I been in a classroom of all high-performing children doing high level work: attention was an issue, for sure, but I am an intensely non-competitive person. I have yet to see a model of gifted instruction that doesn't only pay lipservice to cooperation and multiple intelligences but actually believes in it. I was the kind of kid who threw the county spelling bee once there were two kids left because I wanted it over and 2nd place was pretty good. I am the kind of teenager who never shared her SAT score because all the tension around those numbers was scary. I am the kind of adult who won't play Trivial Pursuit at the Albatross because people get so nutty about winning.
So the gifted classroom was likely to make me sit with a box on my head more than I did already (it's fun to make whoooing noises from under the box! Like your own ghostly echo chamber!). Besides, the stuff I learned and read and did I learned and read and did because I liked it. I am one of those irritating nerds who learns stuff because it's neat. I majored in a field notorious for its geektitude. I went to a university that prides itself on being the place where fun goes to die. Etc.
Also? I don't know about the everyone-knows-the-alphabet Kindergarten, but in my classroom, which annually spans from fluent readers to no-name-writers (somehow people assume there are no high kids on the southeast side, and honestly? Some of my kids are probably higher than the average child at Clarendon), everybody looks pretty gifted to me. If Kindergartners were formally identified for GATE, I'd probably identify everybody, because every kid has some spike of awesome in some area that needs nurturing so they can apply that awesome broadly.