UPDATE scores AS SC
SET ranking = AC.ranking
FROM
(
    SELECT 
         test_code
        ,subject_code
        ,student_code
        ,score
        ,RANK() OVER (PARTITION BY test_code, subject_code ORDER BY test_code, subject_code, score DESC) AS ranking
    FROM scores
) AS AC
WHERE SC.test_code = AC.test_code
    AND SC.subject_code = AC.subject_code
    AND SC.student_code = AC.student_code