Knowing this manga's obsession with demonic numbers, I'm surprised that the demon ranking system don't go all the way up to rank 13. ך (Kaf) for 11, ל (Lamed) for 12, ם (Mem) for 13.
I also think stretching the grade limit a few extra stages would add better progression too. Maybe move some of the characters' (specifically the adults and teachers) existing ranks. Amelie and stay at 6 right now, the other teachers can be bumped up to 8 and 9, while those like Kalego & Balam can be 10, then Opera can be 11, and Sullivan is 12. 13 is obviously reserved for the Demon King.