Nysmith sends more students to TJ every year. It’s very good. Potomac is also a good school. Some Longfellow students turn down TJ to attend Potomac. Flint Hill is also good.
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Nysmith School
Head of School: Kenneth Nysmith – blog@nysmith.com
Religious affiliation: None
Basic Background: Established in 1983
Accredited by
NIPSA (National Association of Independent Schools)
SACS(Southern Association of Colleges and Schools)
CASI [...]

Top 100 N.Va. Elementary School Scores on Standards of Learning This chart ranks the top Virginia elementary schools in the Washington, D.C., area. Ranked schools had complete data for the number of students rating “advanced” on state tests of reading and math.
Rank Elementary School Advanced in 2003 Advanced in 2008
1. McKinley Elementary 49% 79%
2. [...]

Criticalthinking company publishes many books to prepare you for Cogat and NNAT test required by GT screening.
http://www.criticalthinking.com/searchBykeyword.do?code=p&catalog=p&searchKey=Cognitive%20Abilities%20Test&directory=test&gclid=CIX47ZKplJoCFQNfFQodwiF1Lg

The SCAT isn’t used for much.  I think it’s considered rather coachable.  I’ve had my children take it, and qualify for those programs, but I wouldn’t assume anyone would put a lot of weight on it other than for CTY.  It is a single data point.
 For GT identification in school, a more acceptable and usual [...]