Angel is the final school tier, and once you get at least one of the higher awareness angel girls to 5 hearts and max PP you'll unlock the green eraser thing, and to unlock levels 4 and 5, you need to finish with the elf girls, by getting at least one of the higher awareness Elves to heart level 5 and max PP, then the demon girls, by getting at least one of the higher awareness demon girls to level 5 hearts and max PP,