Should I say thank-you or just shake my head in shame?

Take it from her, say nothing , dont even look at her and never speak to her again. Stealing is stealing. I don't buy into that "kids will be kids" idea because teaching her that stealing is alright now will lead her to steal more thinking its ok. Since the school doesnt care ( kinda stupid if you ask me) and your mom doesnt, then you need to punish her in a way. Ignore her and stop being her friend.