Which conference joined in 1954 with a beavers mascot?
CREATE TABLE "old_northern_state_conference_membership" (
  "school_ihsaa_i_dnum" text,
  "location" text,
  "mascot" text,
  "num_county" text,
  "year_joined" real,
  "year_left" real,
  "conference_joined" text
);
SELECT "conference_joined" FROM "old_northern_state_conference_membership" WHERE "year_joined"=1954 AND "mascot"='beavers';