Enrollment at the Martha’s Vineyard Regional High School is expected to drop by 150 students — or about 20 per cent — over the next 10 years, according to a report from the New England School Development Council.

The enrollment forecast, based on a general formula the council applies throughout the region and revealed at a school committee meeting Monday, added tension to already difficult school budget negotiations. The regional school budget, passed last week, already has seen revisions to a multimillion bonding plan and triggered a separate and unusual parent-initiated request to towns for money to fund an arts position.

High school enrollment numbers, at 766 for the current year, will drop to 604 by 2018, according to the report. The numbers, received by Vineyard schools superintendent Dr. James H. Weiss last month, suggest a steady decline to 687 over the next five years, then a leveling out period followed by a further decline.

The forecast for the next four years will be examined by school budget planners over the coming year. Speaking by telephone yesterday, Mr. Weiss explained that the council’s report is an indicator of general trends rather than a firm prediction. “This gets updated every year,” he said. “It points to a downward trend which then levels out slightly in the short term. The data after 2012 is very soft.”

The council uses the cohort survival technique, a research system which factors drop-out numbers, births and deaths and retention of pupils in the same grade and other data into the predictions.

Though no definitive explanation is available for the forecasted drop, theories abound.

At a high school committee meeting on Monday principal Margaret (Peg) Regan suggested that a group of parents drawn to the Island from New York city following the terrorist attacks of 9/11 may now be dispersing. “If you were living on the Lower East Side, this was a safe place for your children,” she said. “That cohort is starting to decline.”

Martha’s Vineyard Commission staff planner Jim Miller pointed to our high cost of living (overall, 60 per cent above the national average). “It’s a great place to raise kids but families now struggle financially,” he said yesterday via telephone. With a national recession looming, that financial strain could become even more relevant, he added.

Mr. Miller also said that while data from the Massachusetts highway department predicts slow upward growth for the Island for the foreseeable future, the growth does not necessarily mean more youth. “Martha’s Vineyard is a place for older people,” he said. “Retirees move here, more than families.”

Also at Monday’s high school committee meeting some progress was made with its ambitious $1.5 million sewage management system proposal, which would connect the high school with the Oak Bluffs wastewater treatment plant.

The committee unanimously approved spending for design and construction of the system, allowing Mr. Weiss to put the issue before the towns. Some committee members remain unconvinced of the project, and Mr. Weiss emphasized that several points in the school’s timeline over the coming months where the project again would be reviewed.

Committee member Roxanne Ackerman voiced concern about expansion projects considering the declining enrollment forecast. “The wastewater problem is based on a certain population of the high school,” she said. “The enrollment numbers impact on the need for this plan. We can’t be building things that we can’t justify.”

Edgartown committee member Leslie Baynes disagreed. “We have to think about the long-term capital plan of the school,” he said.

The proposal, devised in answer to the high school reaching capacity with its current septic system, was removed from this year’s budget amid the uproar over cuts to instruction at the high school and a significant increase in fixed costs. However, following a discussion between Mr. Weiss, assistant to the superintendent for business affairs Amy Tierney and an outside financial advisor, Mr. Weiss returned to the committee with a revised bonding structure to appear in the budget for fiscal year 2010.

Originally the wastewater proposal was twinned on a single bond with funding for a new fleet of buses. Now if the wastewater plan goes ahead, it will be on a separate bond, starting a month later than the bus bond — in July, which is the beginning of fiscal year 2010. Though two separate bonding costs would have to be paid under this scheme, some savings would be made in legal and financial advice. Crucially, in a year with significant budget constraints across the Island, the project would not cost towns in the coming year, though permission to move forward will still be required from four out of six towns. Mr. Weiss confirmed that letters requesting this authority were sent to all towns yesterday.

Meanwhile drama over cuts to the high school arts program is being kept alive by a group of parents who have taken the issue into their own hands. The preservation of music and performing arts committee, formed by a dozen parents of Minnesingers from the high school, is seeking to raise $27,000 from towns to offset school budget cuts to the high school music department this year.

The school committee’s decision to remove half a full-time salary, a measure which will represent the first reduction in force in the high school’s history, was met with anger from some pupils, parents and alumni who see the move as the beginning of an ongoing assault on the arts.

According to member Howard Marlin, the committee has two aims. “Firstly we need to raise awareness of the crisis and second to hopefully raise these funds to save the music department salary. But we’re department-centric and we do not represent any individual teacher.” So far the committee has taken its request to West Tisbury and Tisbury selectmen who agreed to present it as an article on the warrant for annual town meetings in April.

Since a reduction in force clause is still being written into teachers’ employment contract — it remains unclear which position will be affected within the department.

Mrs. Regan said she is unsure whether she can reinstate the position or accept the private tuition.

“It’s crazy to keep classes going if no one’s in them,” she said, “and I can’t personally accept a check for a salary in the music department. Perhaps classes could be arranged independently.”