A slang word for the very powerful prescription drug Alprazolam also know as Xanax in the Benzodiazepine family. They are highly abused and very addictive with a street value of 2$ a milligram.
The term "Yellow School Buses" gets its name from the 2mg XANAX pill which is shaped like a bar sometimes are yellow. The pill can be broken into 4 pieces each equaling .5 milligrams. Hence the name "Yellow School Buses"
Kenny: I woke up in a someones house and cant find my cell phone, wallet or my pant's and don't remember a damn thing.
Ivan: Must of been those Yellow School Buses you took.