Basically, AAC is meant to be used both receptive and expressively. Meaning, the adults use the system to talk to the student (receptive input to the student). And, eventually, the student will use the system to communicate (expressive output).
A visual schedule is used as a receptive visual representation of the verbal/auditory message you're giving them ("hang up your backpack")
Here is a great document:
AAC vs. Visual Schedule