There is a tiny gifted school called
http://feynmanschool.org/.You may want to check it out and see whether it is similar to the one you are in now.
The DC private schools are not famous for rigor in the lower school years. By high school, there are lots of a good options. Montgomery county's Blair high school has a great STEM magnet options. The gifted program in elementary program is supposed to be the top 2-3 percent of the county, more selective than most gifted programs around the region. But it won't start until 4th grade.
