"It would be a marvelous opportunity, she said sarcastically, to forget. To forget who you were, what you were and why you were that. The process, she said, was called assimilation, and that was what was intended for the precocious few who might prove a nuisance if left to themselves" (178).
Assimilation includes allowing someone to change your name because it's easier, to stop using your native language, and the eventual loss of that language, to give up parts of your culture in order to fit in (clothes, ect.). Nyasha fights aainst these things and pays for her stubourness and independence; Tambu is accepted into school and must attempt to assimilate or risk unacceptance.
I believe what your teacher is asking you, is to explain the reasons Nyasha gives to Tambu for her misgivings.