User blog comment:WeissSchnee/New Information/@comment-3146930-20130824015805

Weiss uses Dust, which is more of an "Elemental" based power. She hasn't been shown using Aura at this point in the series, or she just isn't skilled enough.

You also have to consider that the scar is somewhat large, meaning that the cut that caused it was extensive, most likely it was beyond her ability to heal. Even if she learned how to use Aura at Beacon, scars don't heal.