Might I also point out, Arnold didn't want Fiona as the new teacher. Fiona never actually duped Arnold, she talked to him about it, after they came back from the islands the first time.
Also, Carlos is a good kid, who loves his little brother, as any big sibling would. Therefore, he would not be a jackass and tell his own classmates they shouldn't be changing who he is.
Might I also point out, Arnold didn't want Fiona as the new teacher. Fiona never actually duped Arnold, she talked to him about it, after they came back from the islands the first time.
Also, Carlos is a good kid, who loves his little brother, as any big sibling would. Therefore, he would not be a jackass and tell his own classmates they shouldn't be changing who he is.