(CNN) — The nation’s public education system could be transformed under President Joe Biden’s plan to create free, universal pre-K, which would make preschool available and affordable for 6 million more children.

But if passed by Congress as part of a sweeping social safety net bill, one major challenge to implementation is hiring tens of thousands of new teachers at a time when schools are already struggling to fill existing positions.

Between 40,000 and 50,000 new teachers are needed to enroll just 70% of 3- and 4-year-olds, according to an estimate from the National Institute for Early Education Research at Rutgers University. That doesn’t include any additional classroom assistants, paraprofessionals or other staff that might be needed.

Hiring so many teachers and staff is going to take time. Some states and localities that currently fund some form of public preschool, like West Virginia and Washington, DC, already report teacher shortages. The average number of college graduates who completed teacher preparation programs fell 24% between the 2009-10 and 2018-19 academic years, according to the American Association of Colleges for Teacher Education.

Plus, some K-12 districts are struggling to fill open positions, ranging from teachers to substitutes and bus drivers — a trend that predates the pandemic.

“We don’t really have a pipeline of early childhood educators who are sitting idle,” said Chad Aldeman, policy director at Edunomics Lab, a research center at Georgetown University.

“If the federal dollars boost up pre-K enrollment overall, that could lead to a hiring crunch, potentially leading to shortages and a surge of novice and uncertified teachers, especially in low-income communities,” he added.

The proposal, which is included in a Democrat-backed $1.9 trillion spending bill moving through Congress, attempts to address those quality problems in the long run by requiring new pre-K teachers to earn a bachelor’s degree within six years.

The bill aims to enroll more children and increase the quality of pre-K programs. It would also limit overall child care costs for families with children younger than age 6 to no more than 7% of income for those earning up to 250% of state median income, expanding access to about 20 million children. Together, the bill would provide $381.5 billion for those provisions over six years.

Limits of state-funded pre-K

The Democrats’ push for universal pre-K and affordable child care comes as the economic effects of the coronavirus pandemic continue to hold some families back. In some states, low-income families qualify for affordable pre-K programs, but there are big gaps in coverage.

Forty-four states and Washington, DC, provide state-funded preschool programs, but most do not have a seat available for each 3- and 4-year old.

The programs vary widely. A majority have an income requirement and many don’t provide a full day of instruction. Several of the pre-K programs rely on mix of funding, including federal funds from a program called Head Start.

Nationwide, nearly 34% of 4-year-olds and about 6% of 3-year-olds were enrolled in a state-funded program during the 2019-20 school year. Together, that totals more than 1.6 million children, which does not include students enrolled in privately run programs, according to the National Institute for Early Education Research.

“Almost all of the problems come down to money, which is why this federal approach is likely to work,” said Steven Barnett, senior co-director of the institute.

“But the big challenges will be developing the workforce and creating the infrastructure to support them,” he added.

How long will it take to get universal pre-K?

Universal pre-K is not something that can be built overnight. States will likely be responsible for designing their programs and distributing the federal money — a task that could move more quickly in places where there is a robust state-funded preschool program in place.

If the bill becomes law, the federal government would cover the cost of the pre-K programs for the first three years. States would be required to pay for a portion of the cost for the next three years, with the percentage increasing each year until the state is paying for about 36% of the cost in the sixth year. The bill only provides funding for six years, leaving states completely on the hook after that unless Congress reauthorizes more spending in the future.

It’s unlikely that each state could serve every 3- and 4-year-old in six years, Barnett said, but added that the federal funding would still significantly speed up the process.

“Without this, I don’t think we would have universal pre-K in this century,” he said.

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