DD went to pre-K, and the teacher promptly admitted to my wife that she had nothing to teach her. But DD enjoyed the school experience, and fit in very well with her classmates socially, who even openly admired DD for her brains. The teacher went out of her way to help my daughter shine, even allowing DD to read aloud to the class instead of doing it herself.
At the end of pre-K, the teacher was emphatic that she should skip K and go straight to first grade, and was prepared to recommend the same to the school administration, except we were switching states (TX to LA).
Once we got to LA, there was a whole different mindset. There's a gifted program available, but it's closed to kindergarteners unless they test in the HG range (DD is MG). They kept trying all these patchwork solutions until she reached 6yo and could be accepted into the gifted program, but the patchwork solutions weren't working, DD was learning to hate school, and was dumbing herself down... this child whose beautiful penmanship at age 3 was beyond any I have created to this day declared that she'd forgotten how to write 't's.
We began to argue for her to be skipped to 1st grade, and the school refused to even listen to us, so to do so we'd have had to take it all the way to the district superintendent. By this time it was becoming obvious that the staff had decided to internalize the issue and make it about themselves instead of my child, so we pulled her out and homeschooled her for the rest of the year instead. She recaptured her love for learning, and even started flirting with 3rd grade math.
Now she's back in the same school district, though thankfully in a different school with a different staff. She's still in 1st grade, and administrators keep saying nonsense about "gaps", as though this is something that can't be measured and can only be feared. She's now in the gifted program each day for math and language, and she loves that. It's the rest of the day in her regular 1st grade class that's a problem. When she's getting math in GT and then getting the more boring math again later in the day in the regular class, that's a problem. When her regular class is out doing PE while she's in GT, that's a problem... especially when this whole "age grouping" nonsense is supposed to be for socialization, and isn't PE an important social activity?
Life would have been so much easier if they'd just listened to us and skipped her ahead... but with the gifted program being such a political football in the school district these days, I think they served her poorly because this was an obvious candidate for those services and she'd be another number they could use to help justify its existence. If she's half the kid we think she is (and more importantly, the one the tests say she is), she'd have ended up in the gifted program in another year or two despite being skipped.
/rant
