Now that my daughter is in sixth and we have been in two (drastically) different schools, my advice is to really look at the climate.
In our current school district, sixth grade girls are dating ninth grade boys (on a regular basis). My daughter is both appalled and feels left out at the same time. She is only slightly younger. Luckily, our school district allows for her to take accelerated subjects. However, there is very little social studies or science until middle school.
At our last school, most of the students were way above grade level (not considered GT). My daughter was reading three years above and she considered herself a "bad reader". She totally fit in with the group and they had very stimulating conversations. They had two hours of homework per night in fifth grade and they were able to finish way more of the curriculum and beyond. They also completed a full year of social studies and science.
This varies from school to school, city to city, and state to state. Unfortunately, Colorado is a local control state, which means that each district sets the curriculum. In our town, they are forgoing the science and social studies to concentrate on testing areas. And these subjects are usually to help with reading. Last year in American History, they studied until the Revolutionary War. We had quite a bit to learn in the summer.
