B’’H Shalom. I am currently a highschool student (non-Jewish) and I have a friend who is a self mutilator. This person typically uses blades to inflict wounds on their wrists, shoulders, etc. When I found this out, I started taking that person’s blades, usually against their will. I want to know, is this considered stealing, according to halacha, or is it considered pikuach nefesh?

If this friend of yours is damaging himself (and it surely sounds like it) you are doing the right thing. I think it will be a good idea to speak to the school's child welfare manager or the principal to try to treat this child professionally.