Your instructor has used an interesting "hook" to encourage you to examine whether John Updike's "A & P" meets his own criteria for a good short story. Updike is a masterful story teller, so, in theory, "A & P" should reflect the ideas he has about a story's structure. Although we cannot write your essay for you because that would would be a disservice to you, your fellow students, and your instructor, we can guide you to an analysis of "A & P" that will allow you to answer the questions about Updike's success in modeling his story on his theoretical construct.
First, as I'm sure your instructor has discussed, "A & P" is a coming-of-age story—that is, it describes the evolution of a teenager from a simplistic view of life to a view that reflects a more complex and more sophisticated analysis of life and its elements. Often, the consequences of an action become the focus for the character who has the experience. In general, a coming-of-age experience is dramatic because it creates a sudden change in perspective, so one issue you need to analyze and resolve is whether the main character, Sammy, has such an experience. Second, you must decide, based on the story's narrative, what effect, if anything, the coming-of-age experience has on Sammy. In other words, what changes take place in him that reflect a life-changing experience, and does he recognize that his life has changed in a significant way?
Because your assignment is to assess Updike's success in structuring his short story on his own ideal construct for a short story, you need to analyze whether or not "A & P" engages you from the beginning, Updike's first criterion. I would suggest that an effective way of engaging the reader is to place the reader in the mind of the main character rather than through exposition--that is, simply describing a scene as if with a video camera. What is the effect of allowing the reader into Sammy's mind at the very beginning of the story? Because the story is about Sammy's change, what is Updike's goal in giving the reader unrestricted access to Sammy's thoughts and feelings?
As your instructor notes, Updike believes that the ideal story's middle section should "sharpen my knowledge of human activity," so your goal is to explore Sammy's interaction with other characters—Stokesie, Mr. Lengel, and the girls, especially Queenie—to see whether Updike is expanding the story beyond Sammy's internal dialogue to include his interaction with outside forces. What is the nature of his relationship, for example, with Mr. Lengel, a classic authoritarian figure, and how does that relationship change in response to Sammy's identification with the girls? Is any change in Sammy indicative of the movement toward his eventual coming-of-age experience, or are his hormones driving his actions?
The ending of the ideal Updike short story should give the reader a "sensation of completed statement." Certainly, Sammy's comment that "I felt how hard the world was going to be to me hereafter" has the feel of an epiphany, a coming-of-age realization that his life is not going to be as simple as it was before. But you need to question whether or not Sammy's comment truly reflects a new and permanent perspective or simply reflects the realization that he's going to have some additional, but temporary, problems such as explaining his actions to his parents.
In sum, then, your essay should explain whether or not the beginning engages your attention and why; explore Updike's success in thoroughly describing Sammy's relationship with the girls and other characters and how those relationships reflect changes within Sammy; and, last, answer the question of whether the ending provides the reader with a sense of certainty about Sammy's reaction to the entire experience.