For years, nearly 40 states plus the District of Columbia have used federal funding to help low-income students cover the cost of taking Advanced Placement and International Baccalaureate tests.

But, thanks to a big change to the program under the Every Student Succeeds Act, states and districts will need to think hard (and early) about whether—and how—to continue covering those costs.

Some background on how ESSA handles these tests: ESSA consolidated dozens of federal programs, including some that hadn’t been funded in years. One of the programs that had still been receiving federal funding when ESSA passed was the “Advanced Placement Test Fee Program,” which helps low-income students cover the cost of taking advanced exams. The program is currently receiving $28.5 million annually. But next year, it will be part of the brand new, larger Student Support and Academic Enrichment Grants.

The Student Support and Academic Enrichment Grants are essentially a giant block grant that goes out by formula to school districts around the country. Districts can use the funds for a wide range of activities, including technology, student health, arts education, school safety, counseling services, and yes, college-and-career readiness, including, if districts want, AP and IB fees for low-income students.

ESSA recommends more than $1.5 billion for the whole block grant. But right now, it’s on pace to only receive a fraction of that: It gets just $300 million in the Senate Appropriations bill. And President Barack Obama’s proposed budget also gives it much less than ESSA’s recommendation. So there’s a good chance districts will have to pick and choose what they want the funds to go to.

Here’s how AP testing fees for low-income kids work now: AP tests cost about $91 per exam, while IB costs about $147 per exam. The College Board, the nonprofit organization that administers Advanced Placement, picks up part of the tab for its testing fees for low-income students, providing some $30 million of its own money. That brings the cost down to about $53 per test, for AP.

A handful of states use their own dollars to pay the remaining amount, while the majority of states use federal funds to cover much of the cost. Some states then kick in their own money for the rest, or ask families to contribute a small portion (say, about $15 per exam). States and districts can continue to do this, but now AP testing fees will be competing with the many, many other potential uses for the block grant.

And there’s a twist, for next year only: This gets complicated, but ESSA has a special, one-time rule in place for states and districts that allows them to use their block grant funds to cover fees for AP and IB tests taken during the 2016-17 school year only. (Typically, districts must essentially decide what to spend the money on before the school year gets started.) At the same time, though, states and districts that want to continue covering the testing fees have to plan for the 2017-18 school year.

In other words, states and districts essentially have to find double the money, just for this one year.

“That is a big change and it will require states and districts to plan in different ways,” said Trevor Packer, a senior vice president at the College Board who heads up the AP program.

There are a couple of other complicating factors, here. First off, decisions about how to spend the block grant are supposed to be made on a district and not, state level. So districts would have to get on the same page as their states.

What’s more, there may be other funding within ESSA that states could use for the fees. For instance, states have the option to set aside up to 3 percent of their Title I money for direct student services, like tutoring. AP fees could be covered using those funds. But it would come at a cost: Districts would get less Title I money overall.

How many kids are affected? The situation may be tough to wrap your mind around, but it doesn’t effect a trivial number of states (or students). The federal program funded about 880,000 exams in 2016.

And nearly 40 states plus some federal territories used the federal funding to cover at least part of the cost, along with the College Board’s contribution. Those states include: Alaska, Alabama, Arizona, California, Colorado, Connecticut, Delaware, Hawaii, Iowa, Idaho, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Maine, Michigan, Missouri, Mississippi, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Virginia, and West Virginia, as well as the Virgin Islands and Washington, D.C.

There are a handful of states that will be unaffected by the shift, including Florida and South Carolina. That’s because those states already use their own money to pay for AP fees for low-income kids.

The College Board is trying to get the word out about the change. And the organization is encouraging districts and states that see AP as a priority to do their thinking and planning early—even though it may be a while before they know exactly how much money they are getting through the block grant, Packer said.

And, he said, the number of disadvantaged kids taking AP classes—and earning college credit—has increased significantly since the federal program began in the late 1990s. (See chart above).

It would also be helpful, he said, if the block grant received something closer to the more than $1.5 billion level that ESSA recommends.

“I think we are concerned about the difficult choices that will need to be made,” he said. “We are worried [the change] could result in some reduction of opportunity for students.”