Highest value of a row in MySQL
Use the GREATEST() function on all the columns

SELECT
GREATEST
(bangla_1st,bangla_2nd,english_1st,english_2nd,math,social_science,religious,science)
HighestValue
FROM class6A
WHERE roll = 4;


If any of the columns are NULL, then make the NULL the value 0

SELECT
GREATEST
(
    IFNULL(bangla_1st,0),
    IFNULL(bangla_2nd,0),
    IFNULL(english_1st,0),
    IFNULL(english_2nd,0),
    IFNULL(math,0),
    IFNULL(social_science,0),
    IFNULL(religious,0),
    IFNULL(science,0)
) HighestValue
FROM class6A
WHERE roll = 4;


GIVE IT A TRY !!!

In the future, you may add another subject to or remove a subject from the student record.

Here is another method using Dynamic SQL that figures out the subjects present in the table

SET group_concat_max_len = 1048576;
SELECT GROUP_CONCAT(CONCAT('IFNULL(',column_name,',0)')) INTO @grades
FROM information_schema.columns
WHERE table_schema=DATABASE() AND table_name='class6A'
AND column_name NOT IN ('roll','student_name');
SET @sql = CONCAT('SELECT GREATEST(',@grades,') HighestValue FROM class6A WHERE roll=4');
PREPARE s FROM @sql; EXECUTE s; DEALLOCATE PREPARE s;