In the Hunger Games books and films, the nation of Panem is split rather sharply into two:
the Districts, populated mainly by underfed and overworked people who spend their lives mining coal (12), chopping wood (7), toiling in the fields (11), making electronics (3), etc.
the Capitol, populated apparently by overfed and indulgent layabouts who spend their lives gossiping about fashion and watching the Hunger Games on television.
That gives us the working class and the upper class. What about the middle class? Every empire needs its bureaucrats, administrators, people in offices doing paperwork. Where do such people come from? This demographic doesn't seem to fit either the Capitol or the Districts.
Let's exclude (Catching Fire / Mockingjay spoilers)
District 13, which seems to be an entire mini-society of its own, incorporating workers, soldiers, administrators, and a president, and organised on a militaristic basis. For the first two books/films at least, the Capitol and the other twelve Districts function as a country by themselves.