Final answer:
Tracey was blackballed because she broke the rules of the group.
Step-by-step explanation:
Tracey was blackballed because she broke the rules of the group. As mentioned in the text, Bernice wonders why she never receives attention when she is away from home, and attributes it to the unscrupulous behavior of other girls. This suggests that Tracey may have done something to violate the group's rules or betray Millicent, causing her to be blackballed.