Translation:My sister goes to primary school.
From primary to high school, it's from 1 to 6, although 6 should be rather given only for some special achievements - knowledge from outside the curriculum, taking part in some inter-school contests, etc. So usually 5 is the top grade, and obviously 1 is the failing grade.
In university education, the 'old' system stayed (it was like that in schools as well until 20-something years ago I guess): from 2 to 5, with 2 being the failing grade.
Pluses and minuses (and sometimes even 'double minuses': 4=) are present in schools, but the final mark at the end of the year has to be an integer. While at university, there are rather no minuses, and pluses are usually called 'halfs' - and these can stay as the final mark, so you can get a 4,5 at maths at the end of the semester.
this is almost the same in Russia: from 1 to 5 but 1 is given rarely so it's usually from 2 to 5
Actually, this question is a little ambiguous... "Grades" (in Am. English) can also be taken to mean a pupil's age, expressed in terms of the level of progress made through the school system.
I think you can also use it without the 'a'> 'My sister goes to primary school' meaning the level of education she is having and not the kind of school she is going to
I'll second this.
"A" (considering this sentence uses chodzić) would suggest the sister is routinely going to a random primary school every day.
It should really skip the article in the answer.
"a" doesn't necessarily suggest that. It could be the same school, just not a specified one.
It can be worded the same way (and it's accepted), but if you want to be unambiguous, it's safer to add the 'on foot' part: Moja siostra chodzi do szkoły [na piechotę/piechotą/pieszo].