Create proof of purchase codes
You can create a new node type "vouchers" and use UUID module to add uuids to these nodes. When the office creates a new voucher it will automatically be assigned a uuid which you can then use as your code to give the student.

When the student shows the store their code the store can lookup that voucher in a Vouchers view using the uuid filter. Then the store can mark the voucher with a boolean "Used" custom field to show they are redeemed.