There's no POP enforcement on the College car. I use that one too, but it's very seldom your allowed to board on the back doors. I've only noticed it rarely during rush-hour at a handful of major stops, and perhaps at other times during rush-hour.
Now, there's nothing stopping a fare inspector observing who boarded at the back doors, and asking for a pass - but I've never seen it happen.
It's only recently they've started letting this happen ... it seems to be a concession Byford and Stintz have let go through, to deal with the very long dwell times at a few stops. At it's at the operator's discretion.
At the same time, I've tried to board the 501 on Queen street at mid-day through the rear doors, and the driver has refused to open them.