Does she only do it when you are talking her to school? Does she like school?
Charlotte (albeit much younger) used to get like that as soon as we got to day care. She hated the place, it wasn't til I sent her to kinder and she adores it, that I realise how much she really hated being left at the day care centre.
Perhaps talk to her about why she doesn't want to walk herself into the schoolyard, she was she says?