In Act 1 of The Miracle Worker, why are the Kellers skeptical about the competence of of Helen's new teacher?

Helen Keller's mother is skeptical upon meeting Anne
Sullivan, because of her youth, but Annie puts her at ease when she reminds Helen's
mother that she'll have plenty of energy to work with Helen, and she also mentions that
she was, at one point, blind, and can relate to Helen.  Annie then gets off to a less
than auspicious start with Helen's father when, at the Keller house, she refuses to let
him take her suitcase upstairs for her, insisting on doing it herself.  The first two
things Annie teaches Helen are to spell "doll" and "cake", but the lesson ends
when Helen throws the doll at her and locks her in the room, refusing to provide anyone
the key.  This necessitates a humiliating rescue of Annie via a ladder brought to her
window.