INDIANAPOLIS — Primary elections can provide unexpected results, as the one May 5 showed for 10 incumbent mayors who were defeated.

But, increasingly, a May election is a good bet for school districts.

On primary election day, 13 districts around the state put 17 separate referendums to voters, asking for raises in property taxes to fund everything from teacher salaries to renovation. Only five of the questions failed.

Purdue University economist Larry DeBoer wasn’t surprised.

The go-to guru on local taxes in Indiana, DeBoer has been tracking referendum votes since 2008. That’s when Indiana changed the way schools are funded and put more decision-making into the hands of voters.

The November general election is a lousy time for referendums, he’s learned. Only 38 percent have passed.

But when asking voters in May for more dollars, school districts get the thumbs-up almost 70 percent of the time.

Schools may choose a primary or general election for a referendum vote. That’s “almost certainly” why there were 18 on the ballot this May, DeBoer said, and two last November.

Two referendums in November 2014 both got beat. Nine of 10 passed the previous May. Some won hearty support — like a 98 percent vote for a request from Barr-Reeve Community Schools in Daviess County.

DeBoer has some theories. The chief one is that people act in their own self-interest. The May primaries, with fewer contested elections to attract voters, do indeed attract more pro-referendum parents who see a small rise in property taxes as having a big payoff.

“The benefits of added school spending on schools and teachers are concentrated on parents whereas the cost — the added property taxes — are spread widely across the population,” he said.

“So if you have no other reason to go to the polls than the referenda, then the folks who are sure to benefit — people with a kid in school — are going to show up.”

November elections attract a wider array of voters, including plenty with no kids in school but who still must absorb the extra costs if their taxes increase.

It doesn’t always work that way.

The Brownsburg Community Schools asked voters to raise property taxes less than 5 cents for every $100 of assessed valuation. The district, with 7,500 students and climbing, was hoping to get $95 million to renovate a high school and build a new elementary school.

It didn’t work. An anti-referendum campaign led by locals painted it as wasteful spending at a time when the Legislature is putting more money in fast-growing districts like Brownsburg’s.

Meanwhile, the small district of River Forest in northwest Indiana, which has a shrinking base of 1,500 students and is facing the threat of consolidation with a neighboring district, was the beneficiary of a referendum launched by residents asking for a property tax hike of 42 cents per $100,000 valuation.

That came at a time when legislators, under a new funding formula, are pulling more money from shrinking districts like River Forest.

“More school superintendents are learning they have to run these things like political campaigns,” DeBoer said.

Goshen schools figured that out. A well-liked superintendent built a team of vocal supporters that drew upon the Chamber of Commerce, band parents, residents of a senior community, and a popular four-term mayor, Allan Kauffman.

As Kaufmann noted, schools supporters had to “become better sales people.”

DeBoer sees evidence that schools are catching on.

“A campaign is a sales pitch,” he said. “You’re saying, ‘Here’s the price; here’s the product.’”

The Republican-controlled Legislature may be catching on, too. This past session, a proposal was filed to ban schools from putting referendums on May ballots. It failed, but backers are likely to bring it up again next year.

Maureen Hayden is statehouse bureau chief for CNHI newspapers. Send comments to [email protected].