I think teachers like to see the kid for themselves first.
I agree with this, and my DD is the kid who sits quietly. Our school spends the first couple of weeks of each year doing assessments, though, and she assesses well.
FWIW, I've given teachers a heads-up about difficulties, but not about advancements. After her grade-skip, she was very concerned about making friends, so we asked her teacher to pair her up with someone friendly, for instance.
It seems like teachers have been happy to discover DD's talents on their own. She's not way out there, though - if she were a September birthday rather than May, and/or hadn't been skipped, or were a kid who acted out when bored, so she needed in-class accommodation right off the bat, I may have approached things differently.
