I recall that the year before finals in law school, ie the one that is basically spent in the library every waking hour, had everyone's eyesight deteriorate by at least +/-1. I am extremely farsighted from childhood, to the extent that new eye doctors or opticians flat out used to tell me "this is not possible at your age", until they actually started checking, of course) and I ended up more farsighted. There must be a stress component quite independent from adolescent growth.