UPDATE
    SCORES AS SC
SET
    PASSED = TRUE
FROM
    (SELECT SUBJECT_CODE, AVG(SCORE) AG FROM SCORES GROUP BY SUBJECT_CODE) SAG
WHERE 
    SC.SUBJECT_CODE = SAG.SUBJECT_CODE
AND
    SC.SCORE >= SAG.AG
;

UPDATE
    SCORES AS SC
SET
    PASSED = FALSE
FROM
    (SELECT SUBJECT_CODE, AVG(SCORE) AG FROM SCORES GROUP BY SUBJECT_CODE) SAG
WHERE 
    SC.SUBJECT_CODE = SAG.SUBJECT_CODE
AND
    SC.SCORE < SAG.AG
;