Professor Dumbledore had not taken away her wand; he told McGonagall he was holding out hope that she would be allowed to return once her parents calmed down. He had warned her, however, that even if she did not return to Hogwarts, she was not legally permitted to do magic. He had warned her, more importantly, that the Ministry would arrest her and snap her wand if she did. (Thinking of this made her clutch it closer to her, frightened.)