I'm putting together a coaching program at the moment along with the next series of Progression videos focusing on learning to foil so this thread has been an interesting read.
From my experience, a larger slower front wing works well for students but in tandem with using a slightly smaller kite - 1-2m smaller than used use on a surfboard. This way its easier for the student to keep weight forward and up over the board, whilst they master riding off the foil.
With less power, they don't need to lend back so much against the kite power, which stops them accidentally reverting to the back foot heavy twintip stance. It's also good because they may have to fly the kite a bit to keep moving off the foil but when they do come to learn to pop up on the foil, they have just enough power to park the kite and not be over powered. This then means they can learn to ride the foil without needing to lend back more, cant the board/foil over and quickly learn to keep their balance as the foil accelerates.
All this also hinges on convincing students to ride slower to start with, learn the new techniques and balance required before trying to go really fast
In reality, there may not be a one method fits all, for teaching foiling at present - there probably isn't a definitive foil/board/kite combo that works for 99% of students. I'm going with the setup I listed above but am ready to put someone on a bigger kite or smaller foil if it looks like it will work for them.