.I have several questions concerning school.
Should school be plural or singular?
Elementary school is from grade 1 to grade 6
Elementary schools are from grade 1 to grade 6
Context will govern which you use. Both are acceptable. The first can be regarded as a general statement, the second as specific.
"In this country's educational structure, elementary school is from....".
"Elementary schools are from grade 1 to grade 6, while secondary schools...."
Which one is the correct way to count grades?
from grade 1 to grade 6
from the first grade to the sixth grade
from first grade to sixth grade
All three are acceptable.
When is it necessary to use the in front of grade
for example which one is correct?
I'm in the first grade
I'm in first grade
Again, both are acceptable.
If I say school levels, in the plural, does it refer to levels such as grade 1, 2, 3, 4 etc; or, levels in different schools or different places.
That will depend on your context - it could be either.