I'm in a similar situation. DD5 is in K and hasn't yet been tested; even if she doesn't test gifted, she's ahead and she also learns quickly.
We're also in a play-based classroom. The big difference I see is that the teachers *seem* to be willing to listen to me on differentiation; time will tell, though, because we're waiting until parent-teacher conferences in October to discuss. My plan is to go ahead and establish early on that she needs differentiation, even though I don't anticipate an issue in K (similar thing - it's play-based and DD just enjoys being with friends -- also, she's not zooming ahead in learning that much these days, compared to her rate of development when she was a baby/toddler). That way, the discussion has started for whenever it does become an issue, whether that's 1st or 2nd grade.
My main worry is that DD is only about 1.5 grades ahead right now in reading/math, so that might not seem like an urgent issue to the school. But she learns fast, so once school becomes more traditional, she's going to get very annoyed with all the repetition.
Our school seems to have some experience with G/MG kids, and I don't think she's going to test higher than that. So I think we'll be okay as long as we're armed with MAP scores (they just finished testing last week) as well as when she tests on the WPPSI, which will be within the month. Their gifted program is only a 30 min/week pullout for K, but at least they do identify that early and I can have the G&T teacher as a resource. I plan to ask if she can do a Singapore math workbook while the kids are on their 'math stations'. Where she is in the workbook is behind what she actually knows (it's like basic subtraction; at home she's doing two and three digit addition as well as basic multiplication) so she can work independently without needing too much help. Luckily, she *likes* workbooks -- she just has stopped wanting to do it at home and has asked to do it at school.
As for reading, she reads in a "bull in china shop" style -- that is, more whole language/sight words than an understanding of how to 'sound it out.' I've noticed the teacher is asking her to write a few sentences during "journal time" about what she's drawn that day (most kids, including her, draw, not write), and that's really what she needs. She doesn't have much practice writing/spelling, and I also think during reading the teacher is giving her harder words and having her sound it out more.
So, your mileage may vary; if your DS is more advanced than my DD, you might have bigger issues later on. I'm hopeful in our case. As many have noted on the board, schools generally can deal with G/MG kids okay; it's the HG+ kids that throw them for a loop. Even if pigs fly and she actually tests HG+, my experience with her seems to suggest she'll be fine in a regular classroom as long I establish that differentiation is absolutely necessary. I do think it'll be more and more necessary as the curriculum gets traditional, because knowing her, she'll start picking things up much faster than she is now.
