Null values in trigger in SQLite [duplicate]

This question already has an answer here:

• Is there any difference between IS NULL and =NULL 6 answers

I am doing the exercise about trigger in SQLite and there is a question which asks me write the trigger. The questions follows underneath:

Write one or more triggers to manage the grade attribute of new Highschoolers. If the inserted tuple has a value less than 9 or greater than 12, change the value to NULL. On the other hand, if the inserted tuple has a null value for grade, change it to 9.

Corresponding to this question, I type the code:

create trigger nine
after insert on highschooler
for each row
when new.grade=null
   begin
   update highschooler
   set grade=9
   where new.id=id;
end
|
create trigger nu
after insert on highschooler
for each row
when new.grade<9 or new.grade>12
begin
   update highschooler
   set grade=null
   where new.id=id;
end

My code is wrong because I typed new.grade=null in the when statement instead of using new.grade is null. Can somebody explain me the difference between these two codes? Thank you.