Here is a theory that is generally accepted around here.
The subtle (non-physical) planes influence, rather than control, the physical planes as they are seen as templates for planes of denser energy. For example, the astral is the template for the etheric, which is the template for the physical. They are seen as separate, though next to each other; thus, one plane of energy does not break the laws of another, as they are in tune with each other.
In your case, one would use Magick (subtle energy) to influence/aid the physical attainement of a scar. The scar would still be caused by a physical action, though the Magick would aid in getting that scar.
Why you want to create scars is beyond me, though. I shall leave the morals and decisions up to you.