Well, the gaggle of girls is about 12 or so minions + Madison,Sophia, and Emma. So between all of them they could have easily saved up the amount of shit needed to fill Taylor's locker with it.
Or they could have stole from the bin in the toilets, but that means they dug through trash during class and stuffed it somewhere for days as the janitor probably would have cleaned up that every day. Unless the janitor was a lazy janitor.
-edit-
Honestly, canon wise the locker never made sense to me. Given how much preparation had to go into the "prank" in the first place, not to mention why the hell would Taylor even use her locker at that point in canon. It was proven to not be safe. Plus that shit smells, why the hell would Taylor open her locker in the first place if some horrid smell was coming from it?
Which is why I tried to explain it in a way it could have happened in the previous update. Perfume to mask most of the smell until you get close to the locker, and Taylor being horridly fascinated enough to want to know what the fuck they did to her locker.