Return flight from Canada w/ int'l layover.
 

I'm flying Vancouver to Atlanta and I have a short layover in Toronto. I know that they have pre-clearance in Toronto but since my flight arrives and departs in the same terminal I was wondering if I have to leave then re-enter to go through pre-clearance or if I would just go through customs in the US?
Thanks

Your flight may arrive into and depart from the same physical terminal, but the YYZ-ATL segment will depart from the transborder area. In order to enter that, you must proceed through Preclearance at YYZ.

For your purposes, there is no logistical way to clear some passengers at YYZ and some at ATL.