When I say These workers are of grades 2/7 I mean the skill level
In my country a worker who has just started work for a company , he will be classified as grade 01 . After a period of time he will be raised to grade 02 , and then grade 03 , 04....and grade 07 is the highest grade .Just as in the army when you first join
It's still not clear to me. Do mean mean that the workers you are referring to can be in any grade from 2 through 7? You are including workers in all those grades?
If so, you can say: These workers are in/from grades 2 through 7.
On the other hand if all the workers in question are in grade 2, and you wish to inform the reader that there are a total of 7 grades,