Thanks Tycho Dodonus and Daffodil Grimstone, these points make sense, she will avoid doing anything that will get her into trouble, I actually forgot about the simple fact that just sneaking out would be against the rules as I had focused on the disguise side.

The time taken to learn someone's form well enough to convince someone else that knows them personally is an important factor. She probably knows her family pretty well assuming a calm emotional state, and maybe "caricatures" of her classmates and professors but not a convincing disguise.