I have never really understood level assignments. In Florida the solo/ensemble list for trumpet runs from grade I through grade 7.
From what I can tell after 35 years of dealing with it how many high C's seems to be the criteria. Hummel (1st or 3rd mvt.) is a grade 4 or 5 I seem to remember.
Musical content is not even considered. Many grade 5's are much easier technically than 4's or even 3's.
I always tried to have a student pick a solo that was challenging but not such to frustrate him/her and not to worry about what grade it was.
Unfortunately unless they play a grade 6 or 7 they cannot go to the state level no matter how well they play. I may be wrong in that grade 5's may now be eligible for state.