It’s back-to-school season, and parents aren’t the only ones spending a small fortune on supplies.

Parents spend about $166 per elementary-school child and around $300 per middle- and high-school child for supplies, based on data from the Huntington Backpack Index. Altogether that’s a staggering $20.4 billion in school-supply spending just by public-school parents—not counting the hordes of extra donations they’re often asked to haul in on the first day of school.

That amount also doesn’t include all the extras parents pay for, including school fees, field trips and gym uniforms, which combined cost more than $100 per child. Additional fees for sports, band, music rentals, and college prep materials cost hundreds of dollars more.

Other school supplies spending estimates are much higher. The National Retail Federation, for example, reports that families plan to spend an average of $685 on supplies this year, totaling nearly $83 billion.

The good news is the cost of school supplies dropped by as much as 9 percent this school year compared to last year. Still, many parents struggle to pay for supplies—and they’re not alone.

Last year “The Panhandling Teacher,” Teresa Danks, made national headlines when she took to the streets begging for spare change to help pay for the classroom supplies she couldn’t afford to buy. Danks ultimately raised $40,000 last year through in-person and online donations.

Nationwide public school teachers spend $1.4 billion on school supplies, averaging around $500 each according to the U.S. Department of Education. Principals also spend about $680 each on supplies for their public schools, adding an additional $48 million to the school-supply kitty.

On top of that, enterprising Bronx High School history teacher Charles Best founded the DonorsChoose website nearly 20 years ago so people could contribute to teachers’ classroom project requests. Teachers at 80 percent of public schools nationwide now post their classroom projects on the site, and since 2000 contributions have exceeded $715 million, including nearly $90 million so far this year.

This means that altogether parents, teachers, principals and private donors are conservatively kicking in at least $26 billion for school supplies—an amount that approaches the annual revenue of some Fortune 100 companies.

But what most people probably don’t realize is that school districts already get more taxpayer-subsidized supplies funding than Facebook makes in a year: over $44 billion combined.

Even after removing all the supplies spending associated with school district administration, overhead, maintenance, transportation, food service and “other” support services, that leaves close to $17 billion for supplies associated exclusively with instruction and student support. That’s about $340 per student, which is $8,000 to $9,000 per classroom.

That funding alone is more than enough to pay for every single item on a typical school-supplies list—enough, in fact, for two of each item on a standard elementary school list—including pricey backpacks and calculators as well as facial tissue, plastic baggies, paper towels and hand sanitizer.

The upshot is many taxpayers are paying twice for school supplies. So, the next time district officials claim the supply cupboard’s bare, parents, teachers, principals and private donors should demand those officials open up the books before opening up their wallets.