Used to go to CCGS. From memory there wasn't a break in the fence near the Art/Manual arts building so you couldn't have dragged someone onto the school grounds there. You'd have to have dragged her about 30 meters up Queenslea drive to the car par outside that building. There were plenty of bushes in that area. I've been thinking about the possible CG grab site for quite a while. Christ Church (the church) is pretty close to the bus stop outside HJ's, maybe only 100m, so if he grabbed her there he'd have had to king hit her or there likely would have been loud sounds of struggle, certainly yelling/screaming otherwise.
The school has changed a lot, that big car park along the side of the road was only built about 2006/7 ish. Until then that side of the school was all fenced off and grass (the long jump pits were along that edge) until the driveway that ran up to the back of the gym. The chaplain's residence was back there too but might not have been occupied in January. That area used to be quite dark and overgrown with bushes. Further up a little Corry Lynn Road would have also been a good grab site. All these places have changed a lot and don't look all that probable now.
Wherever he grabbed them he would have had to strike suddenly and hard, and had them hooded or gagged before they returned to their senses. Anywhere from the church onwards with a violent blitz attack would have been the way to go. SS's last sighting close to there suggests he liked that area.
As an insight to why CG may have been walking:- When taxis get really scarce and demand high your best bet is often to leave the busy area and try to catch one that's driving into the area to look for a fare before it gets there and twenty people try to hail it. Taxi ranks can get pretty nasty late at night, full of drunks with no manners wanting to go home NOW.