The requirement sits in the statute that governs the whole program, in the list of things a parent agrees to when they sign the contract:
“Use a portion of the Arizona empowerment scholarship account monies allocated annually to provide an education for the qualified student in at least the subjects of reading, grammar, mathematics, social studies and science.”
Two words in that sentence do the work. “At least” means five is the floor, not the list. And “a portion” means the requirement is about where some of the money goes, not about hitting a particular number — it is a spending obligation, and it is a term of the contract you signed, not a suggestion.
Nothing here is exotic. Most families are already teaching all five. The friction is in purchasing all five — and that is where the money goes sideways.