Charles Wallace understands that Meg is torn in understanding who she is.  She is not like her younger brothers, the twins,  who fit in comfortably at school, or like the other kids in school.  She is too intelligent and too easily frustrated by the boring rote memorization at school to easily fit in.  Yet she is not as different from the norm as Charles Wallace, who is so different that he just accepts his differences and does not even try to fit in with the norm.  Charles Wallace just accepts the sense of purpose that draws him to Mrs. Who.  Meg can't comprehend this anymore than she can comprehend the ways of the other kids at school.  She does not fit in in either place.  Thus she is not "one thing or another."