On what Date did the massacre result with 21 Injured?
CREATE TABLE "school_massacres" (
  "perpetrator" text,
  "date" text,
  "year" real,
  "location" text,
  "country" text,
  "killed" real,
  "injured" text,
  "additional_notes" text
);
SELECT "date" FROM "school_massacres" WHERE "injured"='21';