Melanie is not worried when people call April "February" because if the kids at school care enough to call her by a nickname, it means she has been accepted.
When April first moves into the Casa Rosada, she acts fake and stuck-up. Melanie, an exceptionally perceptive child, understands that April has a difficult homelife and is upset about being shuttled off by her mother to live with a grandmother she barely knows. Although Melanie herself makes allowances for April's behavior, she is worried that the kids at school will not be so tolerant. April insists on acting as if she, like her mother, is a Hollywood star, and she gets "furiously angry when she (is) teased". Melanie predicts that "to the kids at Wilson, all (that) stuff" would make April appear to be "a smart aleck", and her "wonderful difference" only "kookiness".
April is indeed perceived as "an odd-ball" when school starts, but with Melanie acting as a "go-between" to smooth the waters, the other kids eventually begin to accept her. When "two of the biggest wheels in class" start calling April "February", Melanie knows that "everything would be all right...it (is) teasing, maybe, but not the kind you use on outsiders" (Chapter 6).