then you can directly get the NULL value from database is SQL Statement
CASE WHEN YOURCOLUMN=0 THEN NULL ELSE YOURCOLUMN END
i am not sure which column is showing 0 for ex check the first column
select CASE WHEN l.admission_number=0 THEN NULL ELSE l.admission_number END ADMISSION_NUMBER,l.student_class ,l.student_name,hindi,maths,social,l.telugu,l.english ,
'' as english_1,'' as english_2,l.science, '' as ns,'' as ps
from lkg_to_seventh_marks as l
union all
select e.admission_number ,e.student_class ,
e.student_name ,hindi,maths,social,e.telugu,'' as english,e.english_1 ,e.english_2,'' as science,e.ns,e.ps
from eighth_to_ninth_marks as e