In simple terms, the graduating class class of 2010.
Extended, X-Game is the group of somewhat cult/gang-like Juniors(currently) at Buena High School, which is where X-Game originated. X is the roman numeral for 10, and therefore being connected to 2010, which is why this group is called X-Game. What a fucking joke. Pretty much just a bunch of faggot-ass juniors obsessed with running around with their gay little orange bandanas, thinking they're cool, yelling "X-GAME X-GAME X-GAME". They are all loathed supremely by '09 and '11, and the partially-intelligent members of 2010. X-Game is perhaps even gayer than the class of 2012, but I highly doubt it.
Fag #1: X-GAME!
Me: FUCK THAT X-GAME!
Everyone else: TIME FOR AN ASS WHOOPING!
*Ass whooping proceeds.*