I don't know if this may help with some of the kids struggling - but work
backwards (in a way), start with an image... it is something I did with my
8th graders when we did 3 point perspective -
I copied images from an Escher book - I used an image of his building (3 pt.
per) - pretty big, but centered on 11 x 17 paper - some border around the
image. (if you don't have copy paper that big, run it off on legal or
regular and tape to a larger sheet) -
The kids had to use a ruler to follow (trace) lines - which they discovered
led to the three vanishing points. after tracing on an existing image, it
was easier for them to draw their own.
This may work with pages from an architectural magazine - or one of the
great Dover publications - copy, have them use a ruler to trace lines -
the vanishing points and the relationships between the lines 'appear' -