The description of Helen's fit at the table is excellent because it shows what Annie Sullivan is up against. It also demonstrates that Annie is at least as stubborn and strong-willed as Helen, indicating that Annie will win out in the end. It shows Annie's commitment to Helen because she knows she can't teach Helen language unless she can control her behavior.
At the table, Helen behaves in her usual fashion. She runs around the table, putting her hands in other people's food. She refuses to sit in her chair, eat from her own plate, use a spoon, or fold her napkin. It has been the family's practice to ignore her, but it's really impossible to do. The scene is comical in its description of what happens once Annie asks the family to leave. Annie must repeat again and again and again the behaviors she wants Helen to learn. In the end, Annie is successful, but she looks like she's been in the fight of her life. This scene is a war of strong wills.
Your access to information to answer this question depends greatly upon which version of Gibson's play you are reading. If you are using the acting edition, which is inteded to be used by theatre groups producing the play, chances are there is not a whole lot of direct information about Helen's behavior. Her actions and behavior are rather implied through the dialogue. Acting editions will usually include italicized stage directions, but let there be no doubt that these stage directions are NOT Gibson's words. Rather they are the staging notes kept by a stage manager for the original production.
However, if you are using the reading edition - which you probably are in an English class - then it will likely include author's notes that will more thoroughly describe Helen's erratic behavior, how she quakes and trembles in fear, how she ruins the dinner table, knocks over chairs, etc. If you are reading one of the reading editions, then it must be said that Gibson has gone to great lengths to describe Helen's behavior.