Amid all the uncertainty swirling around the Tisbury School, the school committee this week made one point clear: the students and staff who start classes on Sept. 6 will be the last to use the deteriorating 1929 structure as it stands.

Tisbury voters will be asked to approve an additional $26 million borrowing request for Tisbury School construction, on top of $55 million already authorized, at a special town meeting on Sept. 20.

“We have a sick building,” Ms. Houghton said Tuesday, during a committee meeting held simultaneously online and at the town emergency services facility on West Spring street.

Black mold, lead-tainted water, lead paint and asbestos all contaminate the 1929 school, she said.

“We have families who do not want to be part of a sick building any more. We have teachers who will walk away … There’s no option to put people back in the building,” Ms. Houghton said. “We cannot do this.”

Calling a halt to the school project is also out of the question, she said.

“We have a number of people who have put considerable outlays in place for this project. We have a modular school system that’s already put in place [and] we’re playing $85,000 a month to rent those modulars,” Ms. Houghton said.

“The construction companies and the [subcontractors] on this project have not taken other projects because they have committed to us, and committed to us to their detriment if we were not to go forward with this,” she added.

Cancellation fees would also be costly, on top of the millions already spent on the project, Ms. Houghton said.

Out of the $55 million authorized last year, nearly $18 million has been committed so far, according to a summary she provided Wednesday.

Reducing the scope of the school renovation and addition would have the same effect as cancelling the project, Ms. Houghton said.

“It means a redesign of the entire building … We run the risk of losing the construction companies, the bids that we have, the pricing we have, and it becomes an extreme challenge.”

Should voters fail to approve the additional borrowing, Ms. Houghton said, there is no fallback plan beyond continuing to pay $85,000 a month for the modular school, a facility she said is insufficient for long-term use.

Previously estimated at $53 million, the Tisbury School renovation and addition approved by town voters last year has ballooned in cost by 54 per cent, to nearly $82 million.

“Estimating for construction on the Island is a tricky thing,” said Jon Rich of WT Rich, the town’s construction manager for the school project, at Tuesday’s meeting.

“Ultimately it comes down to how many bidders you have, how eager are they to do the work — wherever it is —and how much competition is breeding better pricing,” Mr. Rich said.

The construction industry is weathering a triple onslaught of supply shortages, higher material costs and a tight labor market, Mr. Rich continued.

Project cost hikes of 20 per cent to 25 percent have become a regular occurrence in mainland Massachusetts, he said, while Martha’s Vineyard adds more layers of complexity and expense.

“This is not coming off as the most appealing place to do work — dealing with logistics, getting materials over there … dealing with the housing costs,” Mr. Rich said.

Tisbury town administrator Jay Grande agreed that the “Island factor,” as Mr. Rich termed it, is a significant drawback to getting construction work done.

“Just in the last year, if you went to the lumberyard, you’re paying twice as much for materials — never mind labor — than you did the year prior,” said Mr. Grande, who is also the town’s procurement officer.

Simply getting companies to bid on town projects has become a challenge, Mr. Grande said.

“We’re lucky if we get [one] bid,” he said.

“Building inspectors, facilities managers, anyone involved with construction projects on the Island knows how difficult it is presently to get someone to do the work,” Mr. Grande said.

A special town meeting has been set for Tuesday, Sept. 20, at the Martha’s Vineyard Performing Arts Center in Oak Bluffs, to ask Tisbury voters for the additional $26 million borrowing authorization. Under Propostion 2 1/2, a two-thirds majority is required for the measure to pass.

Town officials are waiting to hear from the state department of revenue about whether a ballot vote will also be required, in which case voting will take place Tuesday, Oct. 4 at the emergency services facility.

The school gym will not be demolished until after voting is completed, Ms. Houghton said.

Three mature trees outside the gym had been scheduled for removal Wednesday to make way for a security fence, but the work was put on hold after planning board chair Ben Robinson and Tisbury resident Anna Edey protested the plan at Tuesday’s meeting.

In an email sent Wednesday afternoon, Ms. Houghton wrote that the town tree warden, consulting with an arborist, had confirmed that two of the trees are rotted and will need to come down. The third tree will be re-inspected for disease and rot and taken down if it is affected, Ms. Houghton wrote.

The modular classrooms, installed on the east side of the school campus earlier this year, are still waiting for utilities hook-ups, including a new electric line from Eversource that can handle the demand from both the temporary, modular school and the future, rebuilt Tisbury School.

The school will start classes next month in the old building, principal John Custer said at Tuesday’s meeting. A wholesale move to the modular school is planned for November, provided utilities are in place.

A four-week “look-ahead” project schedule from WT Rich is posted, with frequent updates, at tisbury-school-project.com.