Coincidentally my instructing career was ended by my own vision problems, but I don't recall ever being struck by the issue in respect of students. Except when trying to point at a skier down the hill or from the chair, sometimes you'd realise you had a Mr McGoo...
"See that guy over by the trees?"
"Big guy, in yellow and black.."
That's probably because they're trying to muddle by without glasses under their goggles - or using cheap, foggy goggles.
I think with novices and intermediates, their awareness and visual issues don't extend much past the tips of their skis, so depth perception and so on aren't being tested (which is a safety issue in traffic). While more advanced students are learning to feel the snow through the skis.
Sorry for the ramble