I agree it can be cheaper on digital, but maybe it is the manner of the teaching, or the underlying philosophy of "you can fix it in post, everything you need is in the raw file" that is the issue, as these are the bad habits that have to be unlearned with film (and my personal path was film -> digital -> film). I'd rather see an exercise (if one is teaching using digital) involving handing out a very small memory card (say 128 mb) that can only hold a few frames and telling students to shoot with that (no in-camera deletion), as an exercise in developing observation and deliberation skills.