Another interesting thing about punctuation is that you can't hear it. You're right when you say that the choice of student's or students' depends on the meaning, but when we talk — and listen — that funny little curlicue is unnecessary, indeed impossible. Rather, the context conveys the meaning -- though it is, of course, quite possible to be unclear enough in one's communication that context is of no, or too little, help. Another point I would make is that the apostrophe is a recent invention, really only necessary (to the extent that it is necessary) because of the loss of inflexional endings for English nouns. Notice I say nouns, because even now, we usually don't need apostrophes for pronouns (example: yours and whose, not your's and who's). In Old English, the difference between student's and students' was not a source of any confusion: leorning-cnihtes versus leorning-cnihta.

BTW, how many periods should I have used above after Fury?