Basic education consists of more than 1 Grade. Therefore a group of grades forms basic education.
"Grades 1 to 6 are basic education" Which is true because we are not implying any one grade forms all the basic education for kids. (Any and all of the grades teach basic education).
However if we group the Grades does this not now change? If we use "Grades 1 to 6" or "Grade 1 to 6" to denote a part in the educational system. Then they would actually become the name of the group were are describing?
If we renamed the group to First Stage then we would write "The first Stage is basic education".
So, which of the following would be correct? I would suggest all three. The first two are groups and we make a statement about Basic education (it takes place in Grade 1 to 6), but the last is a reference to what the individual grades do.
- Grades 1 to 6 "is basic education for kids"
- Grade 1 to 6 "is basic education for kids"
- Grades 1 to 6 "are basic education for kids"
To me the use of "is" sounds natural as well as correct, however this maybe some colloquial influence. I have failed to find any reference to the Noun Idiomatically applying to more than one thing remain in the singular for anything unless it relates to a person
Theodore Bernstein, in "Dos, Don'ts and Maybes of English Usage", says that
Idiomatically the noun applying to more than one person remains in the singular when (a) it represents a quality or thing possessed in common ("The audience's curiosity was aroused"); or (b) it is an abstraction ("The judges applied their reason to the problem"), or (c) it is a figurative word ("All ten children had a sweet tooth") (203).