Please,Let me tell you that this apartments can be inside as big as 300 m2 and even it is an apartment , this will have the maid quarters, it is a very common thing in Argentina to have a maid leaving in the home from Monday to Friday or even Saturday. and this is the middle class in Argentina, not the rich one! The rich one has more than one.
Regarding the private schools, you have two types of schools the religion one and the non religion, the last one is more expensive that the religion ones. Maxima was going to religion school a Catholic one, I imagine that her sister went their but not her brother because this is a girls school, so the brother may be went to another Catholic school for boys.
I believe the family had money but not super rich, the father had to support 6 kids, so I guess was not easy. Religious school can give you grants and may be they didn't pay for all the kids, specially if in the case of Maxima we all agree she is very intelligent, she may be was there with a grant.
My brother was in a Catholic school in high school but as my mother was a widow arrive to a point when he was 14 years old she could not pay for the school anymore, she told the priest-director of the school that she was going to take him out and sending him to public school, but the priest allow my brother to be there for free until he graduated because he is extremely intelligent and the priest wanted kids like my brother to represent the school, we are not even catholic we are Danish Lutheran but it is important for schools to have kids that excel so you can be fortunate to study in the best schools without having to be rich.
I understand her father's first wife had money, not him, so may be one of the kids got a grant in the school, who knows, ..... in the other hand Maxima studied in the Catholic University which is private and quite expensive and it is consider better in Argentina a degree coming from Public University than private, the believes is that in public University you earn the degree in private you buy the degree.
I believe Maxima is a very intelligent woman and I would not be surprise if she got some grants along her way.
I apologize in advance if I had been a little confusing in my writing!