Their primary sources for the paper were Titus Livius (whom they found absolutely enthralling to read), Tacitus, Juvenal, Plutarch, Cicero, and Julius Caesar.
The secondary sources were A History of the Romans, by Frank Bourne, Chronicle of the Roman Republic, by Philip Matyszak, Roman Homosexuality, by Craig A. Williams (a scholarly work not in the least a perjorative, but nevertheless it was more than graphic), and Roman Religions, by Mary Beard, John North, and Simon Price. Finally, they did include an abridged version of Gibbon's The History of the Decline and Fall of the Roman Empire. Needless to say, there is also a collection of online sources.
I would call this paper probably equivalent in quality and extent to an upper division undergraduate humanities thesis (although it took them nearly a year to assemble). Sadly, IMO it's what we should be expecting out of high schools.
Sounds like a winner! And I'm young enough that I might still be around to vote for one of them for President. :-)