President Trump’s first budget proposal promised $1 billion in new funds for poor students, with a catch: the money would be used to encourage school districts to adopt a new way of funding their schools.

Tucked into the administration’s “skinny budget,” the single sentence on the issue manages to say a great deal about the Trump administration’s priorities — and about how complicated it could be to move them forward.

The budget proposal calls for expanding Title I with money “dedicated to encouraging districts to adopt a system of student-based budgeting and open enrollment that enables Federal, State, and local funding to follow the student to the public school of his or her choice.” In calling for student-based budgeting, Trump joins a host of big-city school leaders and education reformers who argue that money should follow each student, no matter where they enroll.

It sounds like a simple idea, but it’s far from how most school districts operate.

Districts traditionally create school budgets based largely on how much it costs to pay the salaries of the adults who work in a building. That can mean schools serving high-needs students, which often have less experienced and lower-paid teachers, get less money than schools with more affluent students.

Under student-based budgeting, each student attending a school brings a certain amount of money, which can grow based on factors like whether the student has a disability or comes from a low-income family. That kind of system appeals to those who want schools with greater challenges to receive more funding. School-choice advocates like it, too, since it rewards schools that attract students and makes inequities in funding between district and charter schools more apparent.

It also forces districts to do the student-by-student calculations that could enable private-school vouchers — making student-based budgeting a gateway policy for voucher advocates such as U.S. Education Secretary Betsy DeVos.

Districts such as New York City and Denver have shifted toward student-based budgeting in the last decade, as their systems of school choice have grown more robust. Other districts, including Indianapolis Public Schools, are making the shift now, sometimes painfully. (The approach is also known as fair student funding and weighted student funding.) But districts don’t control the distribution of federal funds, so making the change requires maintaining different budgeting processes.

“There’s a lot of interest in being able to use federal funds in the mix,” said Jennifer Schiess, a policy analyst at Bellwether Education Partners.

But there’s a hitch in the budget proposal: Federal law spells out exactly how Title I funds must be distributed, through funding formulas that sends money to schools with many poor students.

“I do not see a legal way to spend a billion dollars on a incentive for weighted student funding through Title I,” said Nora Gordon, an associate professor of public policy at Georgetown University. “I think that would have to be a new competitive program.”

There are good reasons for the Trump administration not to rush into creating a program in which states compete for new federal funds, though. For one, advertising the new funds as part of Title I, even if ultimately adding them there would be tricky, brought the administration some of its only positive spin on the budget news, which was widely panned as working against poor Americans.

Plus, creating a new program would open the administration to criticism of overreach — which the Obama administration faced when it used the Race to the Top competition to get states to adopt its priorities.

One pathway to encouraging student-based budgeting already exists in the law. When they rewrote the law last year, lawmakers included a pilot program designed to let 50 school districts change the way they hand out funds, including Title I funds. Martin West, an associate professor at the Harvard Graduate School of Education, says he expects a DeVos-led education department would push for that program to expand.

And a big question with any of the possible changes, he said, is how poor students fare.

“It will be important to see the details of what is ultimately proposed in order to see if it’s done in such a way to benefit the students that Title I is designed to serve,” West said.