Around here, 1st grade is the "learn to read" year, so I wouldn't worry about sending a child who isn't yet reading well up to 1st grade. It would probably be a pretty good fit. But if K is the big "learn to read" year in your school, then I might think twice about acceleration.
I also wouldn't worry too much about sending a 17yo to college. If it's a huge concern, you could always do a planned de-acceleration in high school, when there's more challenge and perhaps even free college credits available. Or she could take a year off to work or travel or attend community college. But it always seems silly to me to worry about what will happen 10 or 12 years from now with kids like these. Sometimes I can't figure out next month!
Take the problems as they come and try to solve the ones that are right in front of you without stressing excessively about problems that might never even become problems, you know? I'd send a mature, self-assured 17yo off to college before an immature, insecure 19yo any day!
If she does well with older kids, then I wouldn't worry so much about her size, especially since she's a girl. That just isn't such an issue for girls.
As for the "Is she really THAT smart?" issue: I'm not clear on what testing she's had, aside from the math achievement testing that showed her 1-2 years ahead. Did you say she had the WPPSI? Sorry to be forgetful...I have been sick!
But I can't tell if this is a real concern or just GT denial. (It sounds just like denial, but I want to be fair!
)
