DD1 did this too, for the most of last year... If anyone else like her Dad or Family day carer dropped her off she was fine...
So I had to be a little firm and tell her once the bell had gone I would take her into class give her a kiss and cuddle and leave her, most times she was sobbing as I left Heartbreaking but her teacher would call me 10-15mins later to let me know that she settled.