Because the story represents a specific time, when current-day sexism was not as readily identifiable, it cannot be called "sexist" in any malicious sense. Instead, the story shows how an average teenage from that time reacts to the things around him; if his attitudes towards the girls is sexist, it is only because he has been trained in those ideas, not because he is misogynistic. One very sexist phrase stands out; Sammy, the narrator, pauses in his physical appraisal of the girls to think:
You never know for sure how girls' minds work (do you really think it's a mind in there or just a little buzz like a bee in a glass jar?) but you got the idea she had talked the other two into coming in here with her...
(Updike, "A&P," tiger-town.com)
Coming out of the 1950s into 1961, it was still common to assume that women were mentally inferior to men. Even more common was the idea that women operated more on instinct than on rational thought; Sammy's metaphor of a "buzz like a bee in a glass jar" is the idea that women, and especially girls, don't have conscious thought but instead only react to outside stimuli. Sammy is certainly a sexist character; even his pretend chivalry is selfish, and since he is ultimately not rewarded for either his thoughts or his behavior, it is possible that he will reexamine himself in the future.