The Big Twelve Conference is a high school athletic conference in the Illinois High School Association (IHSA), based in Central Illinois. While the name implies that the conference has twelve schools, there are actually only eleven schools currently active, due to other schools either closing, ending offering varsity athletics, or changing conferences. Stephen Decatur High School closed, bringing membership down to eleven. Rantoul High School then left to join the Corn Belt Conference, citing its inability to compete with larger schools.
Mattoon left the Big Twelve after the 2011-2012 season to join the Apollo Conference. The Green Wave, as the smallest school in the conference, had trouble competing against the more urban schools and are closer in size and geography to the schools currently in the Apollo.
Eisenhower and MacArthur will join the Central State Eight Conference for the 2014-15 school year. Both schools are the final two schools from Decatur that remain in the conference, and the member schools of the CS8 are mostly from the Springfield area.
Starting with the 2014-2015 school year, four Peoria schools joined the Big Twelve, with the departure of the two Decatur schools. The four schools who joined are Manual High School (Peoria, Illinois), Peoria High School (Illinois), Peoria Notre Dame High School, and Richwoods High School.