As a teacher at a nearby school in Philly, I might be able to she'd a little light on this. An earlier article- I can't find the link now- quoted another parent as saying that this was the third substitute teacher of the year. The teacher turnover rate is high, particularly in the rougher neighborhoods. It is more difficult to fill an empty position right now because there is a hiring freeze in the district. So this often results in a string of long-
term subs, which is definitely a possibility in this case. So while I agree the abductor likely knew the child would be back in school, the fact that there was a sub is probably just circumstance.
To answer a couple previous questions: this is not a charter school nor may you attend any school you wish without legal reasons or a false address. Perhaps she used her grandparents' address though to be frank you'd be hard-pressed to think of a significantly worse school in South Philly than the one she attended.
Additionally, I am unfortunately not one iota surprised this monster was able to access a classroom without permission.