What is the IHSAA Class for football for the team joining in 1968?
CREATE TABLE "membership" (
  "school" text,
  "city" text,
  "team_name" text,
  "enrollment_08_09" real,
  "ihsaa_class" text,
  "ihsaa_class_football" text,
  "county" text,
  "year_joined" real
);
SELECT "ihsaa_class_football" FROM "membership" WHERE "year_joined"=1968;