Thirteen-year-old J.P. comes home early from school. Typically, after a full day of classes, he decompresses in his room for an hour but this day was eager to chat about the project he was working on.

“We were using a propeller, but it turns out gears work much better,” he said.

J.P. and his classmates built solar-powered cars and raced them that morning.

“It turns out our car was the slowest,” he said, waiting for a laugh. “And that’s why I said, ‘We should have used the gears.’”

J.P. has autism.

“He’s high functioning socially and behaviorally now, but he wasn’t for a really long time,” said his mom, Jennifer Curran.

J.P.’s reading was “very, very delayed” and he has other learning disabilities. He had violent outbursts early on, Curran said, because he was non-verbal and couldn’t communicate.

With lots of help from the school, Curran said her son is now thriving.

Among other things, he receives speech and occupational therapy. He goes to a “social emotional learning” center at the school where students can get help regulating their emotions. He’s had his own personal aide since he was in kindergarten.

Massachusetts has a higher incidence of children like J.P. than in other states. Children with multiple disabilities make up .5 percent of the school population, while the national average is half that, according to a state report. That’s one reason special education costs are soaring. The law requires schools to pay for these services, so schools look to other programs to cut. Districts have cut sports, music, after-school programs, even teachers.

So why are there more of these students in Massachusetts than in other states?

“More low-birthweight babies survive, and many, but not all of those kids do have disabilities,” explained Thomas Hehir, an education professor at Harvard, who studied the state’s special education population and services.

They survive at higher rates in Massachusetts, Hehir suggested, because of our state’s health care system.

“People generally have access to good prenatal care and neonatal intensive care units,” Hehir said.

Educating kids with multiple disabilities can cost a lot, Hehir says.

It’s hard to get detailed data about special education costs in a district. In the schools where J.P. studies in Granby, Mass., the district spent $1.2 million to educate 13 of its students in special education during the 2017 – 2018 school year, according to state data. That’s about 13 percent of the school budget that year.

This wasn’t a cost lawmakers expected when they came up with a formula for redistributing state tax revenues to less affluent districts 25 years ago. Lawmakers haven’t updated that formula, and in that time special education costs, along with health care costs for school staff, have skyrocketed.

The gap between what the state reimburses districts for special education and what it actually costs amounts to $1.2 billion, according to the Massachusetts Budget and Policy Center.

Districts with strong tax bases can make up the difference on their own, but less wealthy districts have had to cut programs, said Noah Berger, the center's president.

“We see that districts are spending less on teachers, for example, which means there are fewer teachers,” Berger added. “So that means either larger class sizes or less specialists like art and music, and that has a direct effect on the quality of education that kids can receive.”

That’s what’s happened in Granby, where Curran is on the town’s school committee. In the last 10 years, this small district has cut 25 teachers.

“When you starve a school for that long, and you cut and cut and cut to the point where you’re talking about Granby, Massachusetts having 27, 28 kids in a classroom, this is crazy,” Curran said.

This year, the schools in Granby won’t be cutting teachers. The town government agreed to kick in nearly $400,000 to close the budget gap. Curran and other leaders in Granby hope lawmakers will change the school funding formula so it covers the actual cost of special education.

Our coverage of K-12 education is made possible with support from the Nellie Mae Education Foundation.

This article has been updated.