I applied for a position a grade higher than my current job.

(Grammar Questions I Couldn't Answer)

This sentence comes from the worksheet I prepared for my students on "Grade, Tier, Class".  (Originally it comes from the dictionary, from which I took all my example sentences for that worksheet).

A student asked why "higher" comes after "a grade".  Since it's an adjective, shouldn't it come before the noun it's describing, and not after?

I couldn't answer, so I asked around the office.
My manager said, "Because it's a comparative structure.  I think it's as simple as that." 

Maybe he's right.  We do usually put the comparative after the noun, right?  So we say: "the tall boy" but "the boy is taller than the girl"
But then what happens to the "is" in "a grade higher".

Maybe it's a reduced relative clause--"I applied for a position [which is] a grade higher than my current job." ?