What kind of crap is this? The Soviet Union (USSR stands for United Soviet Socialist Republics) ceased to exist after 1991, when their economy collapsed. Not after World War II. But for the most part, you are right. The USSR does not exist anymore (because they're godless Commies), but the Russian Federation does, and so do all of the former republics. If they are asking for the current leader of the Russian Federation, Dmitry Medvedev is the current president of Russia. But if they're talking about the last leader of the USSR that was Mikhail Gorbachev. If I were you, I would just write it on the homework as Dmitry Medvedev, and if the teacher has a problem with it, tell them everything I told you.
I say write it on Medvedev because when people refer to the USSR, they're generally talking about Russia.