This is an interesting question. I think you are being a little too
critical of Nyasha, Dominic. She does fool around sometimes and does
not act according to what her father wants, but she worked very hard
on the testing and did not defy any rules to do exceptionally well.
Just a guess, maybe since Nyasha had already been to England that they
were looking for different students. The nuns might of thought that
Babamakura had too much power to take advantage of his own daughter.
The fact that Nyasha has been living under Babamakura's reign
for such a long time has taken its toll on her. Tambu has only
experienced it for a short time. Since he is not her father, she is a
little more free than Nyasha is. I do not think Babamakura would come
down as hard on Tambu as he did Nyasha, not that Tambu would ever talk
back to Baba, he would just be disappointed at the opportunity that
she squandered. Nyasha may feel more pressure to succeed than Tambu
does, which could have torn her apart.