Thus, Client 1234 did not show improvement (First: 48, Last: 47), whereas 5678 did (First: 58, Last: 61).
NOTE: I added the TestOrder column to the original result set with "ROW_NUMBER() OVER (PARTITION BY ClientID ORDER BY AssmtDate) AS TestOrder" figuring that I will probably need something like this to determine first and last test.
After some deep thought I came up with a solution and thought I'd share it anyways...
(SELECT B.score FROM #GAF B
AND B.AssmtDate = (SELECT MIN(AssmtDate) FROM #GAF C
WHERE C.ClientID=A.ClientID)) AS [FirstTestScore],
(SELECT D.score FROM #GAF D
AND D.AssmtDate = (SELECT MAX(AssmtDate) FROM #GAF E
WHERE E.ClientID=A.ClientID)) AS [LastTestScore]
FROM #GAF A
Now I have a resultset with ClientID,FirstTestScore and LastTestScore.