I... I'm so sorry.
Okay, that's a bit dramatic. But the picture is not good.
First, some (hopefully) brief context. There was no sex education in imperial Russian schools. After the revolution of 1917 and into the 1920s, the Bolsheviks took several steps to secularize and liberalize marriage and legalize abortion and homosexuality, but there was still no sex education. And if there wasn't any even at that moment, you can imagine there won't be any after Stalin recriminalized abortion and homosexuality and made divorces harder to get in the 1930s.
Under Khrushchev, though, access to information improved. The state began to distribute educational manuals late in Stalin's life, but they mainly focused on moral behavior in marriage until the late 1950s, when they began to include discussions of human anatomy and sexual morality explicitly. However, the anatomy was very vaguely described, and various possible ways and positions to use it were entirely ignored.
So what the manuals focused on instead was morality. Promiscuity, unfaithfulness and abortion (though completely legal since 1955) were decried as harmful to the health of a socialist society, and homosexuality, still illegal, was treated especially negatively, considered a form of degeneracy and perversion whose practitioners aimed to corrupt pure Soviet youth. On the other hand, sexual health and morality were linked to physical health generally. That Stalinist version of social conservatism was still there, in large part, and there was still no such thing as a sex ed class in schools.
There is certainly some variation over the next few decades in the way that morality, ideology, and homosexuality were portrayed and how they interacted in these manuals. Substantively, though, nothing changed until 1983, when a “Hygienic and Sexual Education” course was introduced for students in the 10th class, and another one on “Ethics and Psychology of Family Life” for the 11th class — at least, in theory. In practice, even once these classes were developed, they were hardly taught, and taught poorly when they were taught.
The teachers' manuals for these classes, though written by proper sexologists, were very moralistic. They were admittedly less ideologically charged in their moralism, but that's little comfort, probably. To make it even worse, the teachers themselves appear to have been hesitant, if not outright loath, to teach these sorts of things. And there was, even in these courses, little discussion of sexual anatomy — school biology textbooks illustrated the reproductive system with rabbits.
So you're a teenager in the Soviet Union in 1980. You may well have an opportunity to read one of these manuals, though if I'm entirely honest, I'm not sure how it would wind up in your hands. Plenty of copies were published, but I don't know how they were distributed. Either way, though, what is much more likely is that you'll find out about sex through conversations with friends, or through experimentation, and it will almost certainly be unprotected.
Sources:
Alexander, Rustam. "Sex Education and the Depiction of Homosexuality Under Khrushchev." In The Palgrave Handbook of Women and Gender in Twentieth-Century Russia and the Soviet Union, ed. Melanie Ilic. London: Palgrave Macmillan, 2018.
Goldman, Wendy. "Working-Class Women and the ‘Withering Away’ of the Family: Popular Responses to Family Policy." In Russia in the Era of NEP: Explorations in Soviet Society and Culture, eds. Sheila Fitzpatrick, Alexander Rabinowitch, and Richard Stites. Bloomington: Indiana University Press, 1991.
Kon, Igor Semyonovich. The Sexual Revolution in Russia: From the Age of the Czars to Today, trans. James Riordan. New York: Free Press, 1995.