The workaround used on Draco could credibly be the work of a hogwarts student. That was needed because it was intended to be blamed on Hermione. The hit on Hermione did not have that constraint -it is blatantly, obviously, undeniably, the work of a master wizard. The fact that the wards were fucked with is just one stone in the pile of evidence that this was not the work of a fellow student with a grudge, but the work of someone like Bellatrix, Lucius,Voldemort, or a highly competent wand-for-hire.