You might want to consider Waterfront Academy. It is just opening their middle school and just across the river. waterfrontacademy.org

Burgundy Farm - http://www.burgundyfarm.org/
The New School of Northern Virginia - http://www.newschoolva.com/

Not a private school, but if you are in Arlington, Gunston Middle School has a middle school montessori program.

For a girl - Holton

Which area middle/high schools are recommended for children who are completing their montessori elementary education? Are some schools more accepting of Montessori graduates then others? We are in Nova.