Oracle 18c

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CREATE TABLE my_table ( ID, STUDENT_ID, SUBJECT_ID, CLASS_ID, EXPERTISE_LVL ) AS
SELECT 1, '1AAA', '55FFE', 'CLASS808', 2 FROM DUAL UNION ALL
SELECT 2, '1AAA', '55FFE', 'CLASS808', 2 FROM DUAL UNION ALL
SELECT 3, '2AAB', '49BB', 'CLASS890', 3 FROM DUAL UNION ALL
SELECT 4, '2AAB', '49BB', 'CLASS890', 3 FROM DUAL UNION ALL
SELECT 5, '2AAB', '49BB', 'CLASS890', 4 FROM DUAL UNION ALL
SELECT 6, '2AAB', '49BB', 'CLASS890', 3 FROM DUAL;
6 rows affected

DELETE FROM my_table
WHERE ROWID IN (
  SELECT ROWID
  FROM   (
    SELECT ROW_NUMBER() OVER (
             PARTITION BY student_id, subject_id, class_id
             ORDER BY expertise_lvl DESC
           ) AS rn
    FROM   my_table
  )
  WHERE  rn > 1
)
4 rows affected

SELECT * FROM my_table;


ID
STUDENT_ID
SUBJECT_ID
CLASS_ID
EXPERTISE_LVL


2
1AAA
55FFE
CLASS808
2

5
2AAB
49BB
CLASS890
4