How to find past flight routes between 1977/82 Latin America and the former Soviet Union?
Here are a series of old Aeroflot schedules. In 1977, from Latin America, they flew two scheduled routes


Lima > Havana > Lisbon > Frankfurt > Moscow
Mexico City > Havana > Lisbon > Frankfurt > Moscow


It seems unlikely there were scheduled stops not listed, as if you're including a 1h10m refuelling stop at Lisbon you may as well list them all :-). Note that most of the international flights (perhaps all the long-distance ones?) went into Moscow; there were a few into Leningrad but all European routes.

However, it is possible that Aeroflot provided a charter-type service to collect foreign students (especially if it was some kind of large-scale state-backed scholarship scheme). In which case... the "hopping" route and then to Leningrad (via a fuelling stop somewhere) seems quite plausible, but finding documentation on it forty years later would be incredibly challenging.