By the time OoP takes place, Ginny would be a fourth year, which gives her three years to practice. As I recall, in book three, either Sirius or Lupin said it took the marauders about three years to do it. And Ginny is definitely sneaky. She trashed the Gryffindor common room in her first year without anyone finding out. And she's been using the twins broomsticks without their knowledge. And she lied very well to her mother about all those dungbombs got all over the kitchen door at OoP headquarters. She strikes me as someone the sorting hat probably strongly considered putting into slytherin. She kind of just goes unnoticed mostly and she doesn't seem to mind that.