Try the following:
SELECT STUDENT_ID, Numbers, IF(@marks=(@marks:=Numbers), @auto, @auto: =@auto +1) rank FROM (SELECT STUDENT_ID, Branch_id, Class_id, SUM(Numbers) Numbers FROM quiz_user GROUP BY STUDENT_ID ORDER BY Numbers DESC, STUDENT_ID ) AS A, (SELECT @auto:=0, @marks:=0) AS B WHERE Branch_id = 5 AND Class_id = 1;
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