Also, we are a lower-income family living in a modest house and driving a beater car, with some pretty outside-the-mainstream views and habits and a very strong belief in unstructured play and free time; I worry about transitioning to a school that may be mostly wealthy kids of high-intensity parents.
Are you me? I worry about those things too, especially because my parents moved me from another state to an area such as you described, and it failed big time. BUT, I was also in 8th grade/high school then. Not a good time to move a teenage girl, especially one going through puberty and whatnot.
DS7 went from a middle/upper middle class elementary to now a lower/lower middle/middle class one... there are problems in both schools and with kids/parents of all classes anywhere you go. I have also learned that the issues that I fear for my children are usually out of fear for myself and not them. They adapt very well, especially at that age (meaning I worry about the parents' reaction to me instead).
Make a list of all the positives and negatives and rank the importance of each. You'll figure out the best move.
