The "seating chart" that students assume when a teacher is absent and the substitute doesn't know the seating chart. This usually entails everyone sitting with their friends, which makes the class that much more loud and unmanageable.
Hannah: Hey, Josh, guess what? Ms. Kelvin isn't here, we have a sub!
Josh: Aw, yeah, you know what that means! Sub seating!
Sophie: Why are you sitting in your assigned seat? Come sit here, it's sub seating.