Though it was released before the actual series, in terms of continuity, it would appear to fit between issues #11 and #12 of Generation X because Mondo has joined the team and it's also sometime after Chamber accidentally blew up the girl's dormitory.
Join narrator Jubilee as she introduces the other Gen X kids, their two teachers, and the school itself. It's all done in the form of a letter written to Wolverine. Most of the story takes place when Jubilee reflects on a volleyball game, thus the double meaning of the title "Opening Volley."
"Opening Volley" was originally found in the Generation X Preview. It's a different cover for the same issue, but the Overstreet Fan edition was meant to promote both Generation X and the Overstreet Fan magazine at the same time. Both preview versions were distributed during the San Diego Comic-Con of 1994.