It should be taught from the earliest levels and reinforced on higher level courses. A wave, an errant fin or even your own hose or hand etc can all lead to a mask on the head being lost.
I have a bungee in my left thigh pocket of my drysuit.
Things like spools and a spare mask are clipped onto the bungee. If I lose a mask, I can open the pocket, pull out the whole lot and find the mask easily to unclip from the bungee - all one handed.