I think it depends on the kid.
Mine left us NO DOUBT that he was unhappy in 1st grade, though he appeared quite satisfied with his not-very-academic half-day K program with some in-class differentiation. At the time I (wrongly) assumed that he was MG instead of HG+, so I assumed that his relative satisfaction meant he was well-placed in K. I wonder a bit now, but at least I know that when the fit was REALLY bad--as it was in 1st grade--he is the sort of kid who would let us know that something was wrong.
Some kids just won't let you know that school is way off for them. If they're chameleons who are able to get along well with others their own age and are people-pleasers who like to stay on the teacher's "good kid" list, they may adapt rather than showing that there's a problem. Especially if they don't know that there's any other way for school to be, they may not say, "Mom, I'm bored and underchallenged at school."
I think distinguishing between the child who is truly well-placed and the one who is suffering in silence (maybe without his/her even realizing that being underchallenged is the reason for the dissatisfaction) is one of the most important tasks for the parents of a GT child.
In the interest of full disclosure, I was the chameleon who suffered in silence. One of my biggest worries before this school year began was that DS6 would be, too. I was actually kind of relieved when he started acting out in 1st grade because it meant I wasn't going to have to try to figure out if he was really happy or if he was miserable and just going along. I think making that call can be really hard!
