I keep coming back to this post because I like both dresses, and also because I am having trouble deciding what I think. I know that for me, this dress would not be something I would wear to interact with students, but then I have my own strange ideas about what is and is not appropriate that may not match anyone else's. I do think that sleeveless is a bit risky but last year's dress looks very nice and quite appropriate on you. If it were me, I would tend towards adding a light weight cardigan (short-sleeved, perhaps) or a very light-weight jacket.
I also noticed that my husband, who is an academic as well and fairly stylish, tends to be even more conservative when it comes to the way I dress to interact with students. I asked him recently whether I should wear a tie-neck blouse (with a bit of a v-neck opening, so perhaps slightly revealing but not overly so) to class and his response was "Not a good idea, something more conservative-looking would be better". So I am also wondering how our colleagues (or, as you mentioned, parents of prospective students), who may not be as savvy fashion-wise as YLFabers, react to the way we dress.
Anyway, I think with your dainty figure and your height you will probably look great and work-appropriate in this dress but I can see why you felt the need to ask this question.