Try the following tsql Query :-

DECLARE @ID NVARCHAR(300)= '1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20';
DECLARE @Marks NVARCHAR(300)= '0,70,52,5,8,9,4,6,7,3,5,2,7,1,9,4,0,2,5,0';

INSERT INTO Grade (PersonId,Marks)
SELECT PersonId,
       Marks
FROM
(
    SELECT aSplit.a.value('.', 'NVARCHAR(MAX)') PersonId,
           ROW_NUMBER() OVER(ORDER BY
                            (
                                SELECT 1
                            )) AS row_no
    FROM
    (
        SELECT CAST('<X>'+REPLACE(@ID, ',', '</X><X>')+'</X>' AS XML) AS PersonId,
               CAST('<X>'+REPLACE(@Marks, ',', '</X><X>')+'</X>' AS XML) AS Marks
    ) AS A
    CROSS APPLY PersonId.nodes('/X') AS aSplit(a)
) AS P
FULL OUTER JOIN
(
    SELECT bSplit.a.value('.', 'NVARCHAR(MAX)') Marks,
           ROW_NUMBER() OVER(ORDER BY
                            (
                                SELECT 1
                            )) AS row_no
    FROM
    (
        SELECT CAST('<X>'+REPLACE(@ID, ',', '</X><X>')+'</X>' AS XML) AS PersonId,
               CAST('<X>'+REPLACE(@Marks, ',', '</X><X>')+'</X>' AS XML) AS Marks
    ) AS A
    CROSS APPLY Marks.nodes('/X') AS bSplit(a)
) AS M ON P.row_no = M.row_no;

Result :

PersonId Marks
1        0
2        70
3        52
4        5
5        8
6        9
7        4
8        6
9        7
10       3
11       5
12       2
13       7
14       1
15       9
16       4
17       0
18       2
19       5
20       0

The bulk insert utility is used for importing data from external files, not SQL variables. If you are going to manipulated a large amount of data and you are concerned about performance you can read more about database recovery models where you can change your Full recovery model to Bulk logged for example.

I will advice to talk with superior before changing such settings in your are working in production environment.