Montessori Peace School in Herndon is a good choice as well. My sisters kids have gone there the past couple years and loved it. One of the main teachers used to be with Sunset Hills Montessori for years (she had one of the most popular classes always had a waiting list) but had a falling out with the principal. Its within your time frame and I believe much more reasonably priced than the other options. They do have their prices on their website
http://montessoripeaceschool.com.