The short story "A&P" by John Updike has several topics to consider for a thesis statement. This is a story that combines the challenges of self-esteem, identity, the want to be accepted, the breaking of rules, and the consequences of our actions.
If you were to pick one of those themes, for instance, the consequences of our actions, you could start a thesis statement that could read:
The short story "A&P", by John Updike, treats a challenging aspect of adolescence: Social acceptance versus social choices. The main character is placed in a situation where he can choose to create short-lived reputation in front of his peers versus losing his job as a result of it. When he chooses the latter, he must face the consequences that come as a result of placing social acceptance as the most important choice.
When you think about it, our main character is a young kid who has to work and is not able to enjoy himself during the Summer. As the young girls with bathing suits enter the store, he is placed in a somewhat embarrassing situation where he gets to see his peers enjoying themselves, while he has to work. Somehow, this young man must show that he is also capable of being "cool" by being insubordinate to his manager in favor of the girls. He is fired, and is left with nothing- the girls leave the store as they would have, with or without his scene, anyways.
Yet that tiny moment of control might have given him the only chance to save some dignity. That is what makes all the difference in him, although we know that his choice is rather poor.