With a little imagination (yeah, I know ...) one can make a passable "costume" out of regular clothes and a few carefully chosen toys.
True, but it wouldn’t be “equitable” to the kids who buy elaborate store-bought costumes, and the school is ass-u-me-ing that the kids with homemade costomes will feel inferior.
And the old standby of the “Hobo” costume is so passe these days, you know...