Health and water officials in Tisbury are trying to solve a persistent conundrum: How is lead-tainted water getting into the Vineyard Montessori School’s pipes, when the town’s water system doesn’t use the toxic metal?

“All our water mains are lead free,” assistant water superintendent James Cleary said Tuesday at an online meeting of the town’s water commissioners and board of health.

Polyvinyl chloride (PVC) and ductile iron pipes carry water through the town, Mr. Cleary said, and neither leaches metals.

Yet children at Vineyard Montessori — the Island’s largest child care provider, with more than 75 early-education seats and a summer program — have been drinking bottled water since lead was detected in a kitchen faucet several months ago.

Head of school Debbie Jernegan said she has had all the faucet fixtures replaced as well, in case lead was leaching from older faucets and taps.

Some parents took their children to pediatricians for lead testing, Ms. Jernegan reported, and two told her afterward that their children were fine.

By law, Tisbury health agent Maura Valley said, any elevated levels of lead found in children would have been reported to the town, and she’s had no such reports.

In June, water from a service line feeding the school revealed lead levels more than twice as high and copper levels more than four times as high as anything the state has on record for a school or child care facility over the past six years, according to a letter from town contractor Environmental Partners dated July 21.

“The lead and copper results taken from the service line are exceptionally high at 18.25 and 40.90 mg/L,” Environmental Partners wrote.

“For comparison, the highest lead and copper sampling results available via MassDEP’s electronic data reporting system (statewide data from school and childcare facilities from 2016 – present) are 9.0 mg/L and 9.97 mg/L,” the letter continued.

On Tuesday, town health officials questioned the results of the service line test, in which head of school Debbie Jernegan took water samples using provided equipment and mailed them to a laboratory for analysis.

“A resample needs to be taken…I just don’t have a lot of confidence in that number because it is so crazy high,” Ms. Valley said. “It doesn’t make any sense,”

Board of health member Michael Loberg agreed.

“It’s confounding, is what it is,” Mr. Loberg said.

The town tests its water system for copper and lead levels at about 30 locations every three years, said Roland Miller of the board of water commissioners.

“These are all taken at the residence; these are not samples of the water main,” Mr. Miller said.

The latest round of testing in 2021 found nothing to trigger action under state environmental protection rules, according to the Environmental Partners letter. Earlier testing rounds in 2018 and 2015 were similarly below action levels, the letter continued.

“Results show that the [Tisbury Water Works] corrosion control efforts are effective. The TWW has not made any recent changes to treatment at the well supplies that would affect corrosion control,” Environmental Partners wrote.

While confident the lead is not coming from the Tisbury Water Works, town officials said they’re not giving up on helping Ms. Jernegan find the source of lead and copper in her school’s water.

At Tuesday’s meeting, Ms. Jernegan said she will work with Mr. Cleary, Ms. Valley and a plumber to add a sampling faucet just inside the school building from the service line, following Environmental Partners’ recommendation for the next step in pursuing the source of the lead.

The board took up another Tisbury health mystery Wednesday, but this one seems destined to remain unsolved.

An outbreak of salmonella during an Oak Bluffs School voyage aboard the schooner Shenandoah sickened 11 of the 22 mostly 10-year-old children aboard early this month, assistant health agent Catie Blake told the board.

Three children were tested by Martha’s Vineyard Hospital, which determined all 11 were ill with the same infection.

“I spoke with about half the parents and they were understandably incredibly upset,” said Ms. Blake, who found no clues to the source of the bacteria when she inspected the schooner in port after the trip ended.

“All the food had been thrown out,” said Ms. Blake, adding that she is waiting for test results from a sample of shipboard water.

“There’s nothing [edible] on the boat between trips,” she said.

“It’s really just a guess between … a sanitation practice or actually something they cooked,” Ms. Blake said.

Staff at the nonprofit Foundation for Underway Experiential Learning (FUEL), which owns and operates the Shenandoah, include two staff members with food-safety training, Ms. Blake said.

“All of the things that I’ve asked them to do they have done,” she said.

Nonetheless, Ms. Blake said, she made sure FUEL staffers understood that the outbreak — which took place the week of June 27 to July 2 but was not reported to the health department until July 8 — was a very serious incident.

“This was an unacceptable situation. Kids were poisoned,” she said. “I’m hoping and praying we handled what had to be handled and that it doesn’t happen again.…We’re lucky it wasn’t worse.”

Ms. Blake also had some good news for the board of health Tuesday: The water at all of Tisbury’s public beaches is testing clean, despite water temperatures as high as 83.

“I just checked our [bacteria] counts from yesterday and they’re nice and low, so our beaches and our harbors are looking nice and clean,” she said.