I'm going to approach this from a couple of different angles, bear with me. I have a girl going off to college this month, so I've been coping with some of the same issues.
First, if she's never going to maintain it or sharpen it over her entire school career, you need to take that into account up front. I'm saying super steel here. I'm talking M390, M4, S110V, S90V, ElMax, S30V, VG-10, something that will take an edge and keep it for a good long time and not rust. I love ZDP-189, but my girl got it to rust, so I'm ruling it out. Same same Super Blue, even if your sister liked the look of Spydercos, which I recall you saying she does not. There is a fair amount out there with this level of steel, plenty to choose from. Until you look hard.
Second, replacement. It should be a relatively inexpensive production knife that can be replaced at Christmas or birthday or something. My 0777 M390 Black ain't goin'. A knife like this is as likely to be lost as it is stolen, I think. So, keep the price down and confine your search to current production knives or buy multiples, if you can afford it.
The size thing. Girl pants have stubby little pockets. I have no idea why, but that's the current status quo and my own personal horror isn't going to change that. My girl happens to like the tiny little Kershaw Baby Boa that I got recently for that exact reason. It will sit in her pocket without a problem, even the ludicrously short front pocket; of course, it's a ludicrously short knife. I have a SOG Bluto on the way with a slightly longer blade and a bit better steel than the Baby Boa (VG-10 versus Sandvik 14C28N). Not that there is anything wrong with the Sandvik steel, just that if I knew I had to go three or four years between sharpenings I'm going with a super steel and a 30 degree inclusive edge. My girl carries one Delica or another at work--it's an awesome size for her and the VG-10 blade gets frighteningly sharp and stays that way for a couple of weeks of relatively hard use in the barn (compared to what I do: office ninja).
My other solution to the no-maintenance issue is to go with an H1 steel and a fully serrated blade. Those can be had from Ladybug size on up to my glorious Pacific Salt. A Tasman Salt is a nice knife. The yellow handles are a nice change from the usual black, too. Sadly, those are all Spyderco offerings, so no sale for your sister.
On to blade form. College. My sister graduated only a dozen years ago (or so), and as I recall, she had a bunch of little projects where a knife might have come in handy (she refused to carry one--gun, yes; knife no; I have no explanation). Usually, a plunge cut into cardboard to make stuff. Art, I guess; I only tutored her in math, so I wasn't up on all the other details. My point is that the knife probably needs a pointy point. Your modified sheepsfoot isn't going to be the best plunge cutter available and plunge cutting with what will eventually be a dull blade (by our standards) can be a safety problem. The Leek has a great blade shape for plunge cuts. I'd recommend the G-10 version as the standard stainless steel handle can be a bit slippery and the tip is needle sharp. The Leek can be had in ZDP-189 (if you can find it), but the ZDP will, in fact, rust. I would avoid it in this application. There are two versions with a D2 cutting edge, which is a very decent steel. The 14C28N is apparently relatively more difficult to corrode than the ZDP or D2 but it won't hold an edge as long. The VG-10 Delica never rusts, but it won't hold an edge forever, either. I'm a fan of the VG-10, but it's not S90V.
For simple slicing, I think the Spyderco Chaparral is probably the best blade available. The XHP steel will hold an edge as long as D2 (which I really like) and is much more corrosion resistant. Still, it's Spyderco. <sigh>
The light and handy thing. My girl loves the Eros. It's light and incredibly thin. Very stabby blade, which she likes (I'm not going there). However, the Acuto+ steel, though highly rust resistant, won't hold an edge forever. While it's not a Gerber which gets dull simply by aging, but it's not S90V, either. The pokey tip is awesome, though. I have both the Ripple 1 and Ripple 2, one in ultramarine blue (which is gorgeous) and the other in bronze (also gorgeous). My girl loves those. Very pokey tips, too, but still just Acuto+ steel.
I like the Cold Steel Hold Out III, very grippy G-10, needle point, and a really good heat treat on the AUS-8 steel, but it only comes in basic black. And really, as much as I like AUS-8 because I can get it scary sharp, it won't hold that edge for four years. Not even three. Not even with the girl duty cycle. So, back to the search.
My current candidate is the Spyderco Air. I know, I know, it's a Spyderco, but it's a bit different. First, it is an M4 steel blade. M4 isn't as rust resistant as S110V, but the edge you put on it will be there when she graduates. The blade form is a Wharnecliff with a needle tip. And it weighs less than two ounces. No pocket clip, though. OK, fine. It was a thought.
My other candidate is S30V, which is a pretty good steel, rust resistant and can hold a decent working edge for a long time even with no maintenance. The Kershaw Skyline is now available in S30V, though I don't think it's going to be a standard production item. If you like it, get two, now! Why? Because GENIUS!
If you had two (of whatever knife you choose), you could sharpen one up and send it to her in exchange for the one she's carrying at the time. Annually.
Me? I'm just going to keep on sharpening my girl's knives for her since I'm not sending her anywhere. I'm also curious as to how she will react to the Cold Steel Hold Out....
She can also attach her dorm key or whatever to it as well so she always has it on her.
