When I look on Google, I see both of these used.
... for students grade 6-12
... for students grades 6 - 12
The plural form has significantly more results on Google and sounds more correct to me because I read the sentence with an implied "in" between the words "students" and "grades".
Which form is correct / better (if any)? In particular, if I were to use this on marketing material for an education-saavy audience, which form should I use?