to my mind, the best SD weapons for someone who can't carry a weapon -and which are especially great for students - are the pen and the book.
Pen - You can carry one in your hand anywhere without raising an eyebrow. If you are attacked you can either strike with the blunt end or the pointy end. Stick a pen into someone's thigh and they will probably pause long enough for you to get away, and their speed running after you will be diminished considerably! Plus, if/when dealing with the cops later, you didn't draw a weapon, you were attacked and responded and the harmless pen just happened to be in your hand at the time.
(A flashlight works well as a striking tool but doesn't have the pointy end, and may be less easy whip out. And why would you have a flashlight in your hand during the day? After the fracas, it might look like you were pulling it out as a weapon to use against an unarmed opponent, which is not the impression you want to create.)
Book - another great one for students, natural to carry and no-one can attribute nefarious streetfighting intent to carrying a book. A paperback works quite well, as thick as you can comfortably and securely hold. When reading a book, you hold it with the spine in your palm - so now reverse it so it's spine out, palming the lower corner of the book. It's easy to get into position, and very natural to swing, and when the spine connect with wood siding it leaves a serious dent so it should do just swell against a cheekbone, eh?
Practice a little bit with the pen & the book & you'll see that they can be VERY effective, while also never exposing you to being labelled or punished as "a weapon-carrying psycho just looking for trouble." Plus these are things that you can very naturally carry in-hand at all times, so you aren't put in the position of having to retrieve a weapon from a place of concealment.