This is a draft!
Google, its Structured Data / Rich Snippets tester and schema.org have a semantical bug in the the Rating Schema (imo).
It does not understand, that “grade 1” can be better than “grade 6”, the bestRating value always needs to be a higher value, even if you define those values properly.
Now, of course you can match “grade 1” to “rating 6”, … and “grade 6” to “rating 1” but it has one big disadvantage:
Google likes to see the actual value in the frontend, which means you cannot apply that matching from a SEO perspective.
That’s why you should use Stars instead of Grades (Marks) when implementing a Review Rating.