Time after time, parents step up to make cookies for the bake sale, coach sports programs after school, weed and maintain the school grounds, put up bulletin boards, work the book fair and more.

FORBES: Back to School: Volunteering Your Services

To make things fair, we chose regularly replenished items that all three retailers sell, and we didn't include sale items. (While there are back-to-school sales galore right now, January is prime time for office-supply sales.) To bake in the effect of bulk purchasing, we chose 10 of the largest package of each item.

FORBES: A Penny Pincher's Guide To Office Supplies