According to BoingBoing, all 176 issues of If have been made available via the Internet Archive, including the ones edited by Hugo Award-winning Frederik Pohl. His greatest contribution to the magazine was, perhaps, the introduction of the “If-first” series, which showcased new authors. A number of these writers went on to become extremely well-known, most notably Larry Niven, who published his first story in the magazine.
I seem to always forget how much good stuff is available at Archive.org. These magazines have tons of fiction that could be used in your classroom.