It's very important to realize, in this age when behaviors like this are pathologized to extreme degrees, that however harmless your style of self-injury may be (or even if it's not so harmless), almost every psychologist will over-react to it. That means, hide it if at all possible. If you have any control at all, don't do things that are obviously self-injury and don't damage skin that can't be covered by a T-shirt and shorts. It risks you being hospitalized when you don't have to be.
I wish I didn't have to say that; I wish people didn't overreact; but that's the way it is, and it adds to the problem severely, but there's not much I or anyone else can do about it right now other than informing as many people as we can.
Superficial skin damage is NOT the end of the world. Nor are bruises. Even head-banging can be mitigated to the point that it doesn't cause injury. There are solutions to these things even when they do become dangerous, and the best approach is to look at the problem carefully and find those solutions--not overreact and hospitalize the person.
That said, it is possible, with caution, to get outpatient treatment if necessary for self-injury wounds that have become infected, involve broken bones, or require stitches. In some cases, you may be able to lie about how the injury occurred, but not always. The really important thing is to use your best "rational NT" (or "rational autistic") act, explain that you are not suicidal and that you are working with your counselor on dealing with this, and that you would really like to go home with your family (or your cat, etc.), read a good book, get a good night's sleep, and this will be better in the morning.
Of course, if you do need medical treatment, risking hospitalization is necessary; and one shouldn't be too frightened of it--in general, hospital wards are at worst boring, and if you remember to ignore what the staff thinks of you, they can be survivable.