posted by Casey .
A major reason why children of upper-middle class families do well in school is the pressure that is placed on them by their families.
a I guess..
It's true only if you mean ALL "children of upper-middle class families."
While family pressure could be a variable affecting a child's ability to do well in an educational institution, it is not a clear, direct causal link. A child has the capability to have family pressure, and still not do well in school. A more likely reason for children of upper-middle class families doing better in school is implied in the question, the economic variable.