Isn't the whole point of OO to keep each object distinct and have each object maintain it's own data for greater reuseability?

The functions that manipulate the account data should be in the account class, not the banking system class. Otherwise, what ozzite said.

account
account->setdata()
system->account->setdata()

edit for ()'s, obviously even if it makes no sense gotta do what the assignment actually asks of you.