The Island passed an important hurdle this week on its race to qualify for state funding to replace the aging Martha’s Vineyard Regional High School.

On Wednesday night, a committee of officials from all six Island towns agreed unanimously on a cost-sharing formula for building expenses, a critical element in qualifying for state reimbursement.

“When this goes out for bonding, the voters have the final say. But the capital formula structure is in place and unanimously supported,” said West Tisbury select board chairwoman Cynthia Mitchell, who served as chairwoman of the all-towns formula committee that began discussions in April.

Under the plan approved Wednesday, each town’s share of the capital costs will be based 70 per cent on the number of students it sends to the high school, and 30 per cent on its equalized property values.

But the actual percentages paid will vary slightly, due to what Ms. Mitchell called “a one per cent compensation factor for Oak Bluffs, for . . . being the host town for the high school.”

That amounts to about $100,000 per year that the five other towns will split on behalf of Oak Bluffs, she said.

Wednesday was the committee’s sixth and final meeting, Ms. Mitchell said, and began with a proposal from the previous meeting that calculated the cost per town at 75 per cent pupil numbers and 25 per cent property values.

While most towns were satisfied with that formula, she said, Oak Bluffs — which has long argued that it deserves more consideration for hosting the tax-exempt school — was strongly opposed.

“After spirited discussion we negotiated and agreed on a somewhere in between, which was the 70-30,” Ms. Mitchell said.

The whole process took 40 minutes, she added.

Details of the agreement will be provided in writing by town finance director Bruce Stone, who is developing a report that Ms. Mitchell said she expects will be circulated to committee members next week and then released to the public.

The final agreement will also include a fiscal message to the high school, she said, describing it as a “memorandum of understanding that the high school basically needs to live within the confines of Proposition 2 1/2, just like the towns do.”

Requested by the Oak Bluffs delegation to the committee, Ms. Mitchell said, the draft language indicates that annual budgets from the high school that exceed the state law’s 2.5 per cent cap on year-to-year increases may wind up seeking voters’ approval.

“The towns will frame anything over 2 1/2 in a different format, so it may end up going to a ballot vote,” she said.

The formula approved Wednesday only applies to the capital expenses of the high school project.

The regional high school committee still must resolve the cost-sharing formula for the school operating budget, which is part of the regional agreement first signed by the towns in the late 1950s as they prepared to open the first all-Island high school.

As with the capital formula, Oak Bluffs is expected to ask for a measure of relief from operating costs. But first, the school committee must bring the regional agreement into line with current state regulations, which has required a legal review.

A mutually-supported agreement on capital costs will be essential for the high school to complete its acceptance into the Massachusetts School Building Association’s construction program, which stands to reimburse close to 38 per cent of many costs — the only route to a new high school that could cost $100 million or more.

The school district is working toward a September deadline to fully qualify for the program, which provisionally accepted the high school earlier this year after turning it down in five previous years.