When 72 Island students move to the new Martha's Vineyard Public Charter School this fall, they will take with them $370,000 from the budgets of other Island schools.

This figure is higher than expected, mainly because a key factor -- the average cost of education at existing schools -- has turned out to be greater than anticipated. State estimates of the average education costs at each Island school were released last week.

In some school districts, the impact will be negligible. For example, Oak Bluffs will lose only $27,552, a tiny amount when compared to the town's $10.6 million budget and less than 1 per cent of the school's $2.9 million budget.

But the loss will be strongly felt elsewhere. The Up-Island regional district, for example, will lose $177,379.

"We were expecting [to lose] about $125,000. That's what we budgeted for," said treasurer Seth Mosler, noting that the West Tisbury School had already eliminated a proposed new eighth grade teaching position in anticipation of the cost.

The new estimate "was unexpected news. It will certainly make things much tighter for next year."

The charter school is a new public school, designed by Island residents and approved by the state. The school will rely heavily on project-based learning and other alternative teaching approaches, such as mingling children of different ages. The school will be in West Tisbury.

Start-up costs have been paid for through private fund raising and a $50,000 federal grant. The funding of the school's operation is more complicated, but the burden will fall largely on Island taxpayers. The losses to local districts depend largely on the number of children each district is sending to the charter school.

This funding system has been a concern for some, because when a large, established school loses a handful of students, its expenses don't necessarily go down. Charter school advocates argue that the investment is worthwhile if the charter school is able to provide better education for some students. Different students thrive in different kinds of educational environments, they say.

Here's how the funding system works:

Every school spends a certain amount of money to educate children. The majority of this money comes from local property taxpayers, with some help from the state. State aid comes to the school or town every year on a line called "chapter 70" on a document called the cherry sheet.

For example, in Oak Bluffs, the operating budget of the school last year was $2,545,959. For the same period, the town of Oak Bluffs received $81,610 from the state. A total of 420 children were enrolled.

The state uses these figures to determine the average cost of education at each school, a per-pupil tuition cost. Then, for each student going to the charter school, the state subtracts the tuition costs from the chapter 70 line item on the cherry sheet. That money is instead sent to the charter school.

In the first three years of charter school operation, however, the state will deduct only a percentage of the tuition. Eventually, the state will subtract the full amount.

Next year, for example, five Oak Bluffs children will attend the charter school. The state has determined that the average cost of elementary education in Oak Bluffs is $9,184. Therefore, the state will send $45,920 to the charter school. Eventually, if those children continue to attend the charter school, that full amount will be deducted from Oak Bluffs reimbursement. This year, however, the state will deduct only 60 per cent of the cost, a total of $27,552.

Officials in the town have not discussed how they will handle this loss, but they do not seem overly concerned.

"Twenty-seven thousand dollars in the overall school budget isn't an immense amount of money," said Roger Wey, the board of selectmen's liaison to the school department. "That isn't bad for five kids."

Other Island officials expressed more worry. Superintendent Kriner Cash said he is surprised that the average education costs were judged to be so high by the state Department of Education.

"They are higher than we expected and budgeted for," he said. "These average student costs are much higher than we had thought and it's difficult to figure out how exactly they arrived at these."

Yet another aspect of the equation is the percentage of help that will come from the state. Local school officials had believed the state would pay half the tuition costs in the first year, then learned that the state is planning to pay only 40 per cent. Legislators are lobbying for the additional 10 per cent.

If the Vineyard charter school had opened last year, when most charter schools opened, it would have received the additional money, said Jean Lythcott, president of the charter school board of trustees.

"We are urging people to call their legislators to say they have an interest in this, in passage of it," she said.

As the formula is set up now, with 40 per cent reimbursement, the losses to Island school districts will be as follows:

The high school, which will send six children to the charter school, will lose $46,595 in the first year. Edgartown will send 11 students to the charter school, losing $52,562 on its cherry sheet. Tisbury will send 14 children to the charter school, losing $66,360 on the cherry sheet.

The up-Island district will lose the most because it is sending the most children, 36 of the total 72.

Meanwhile, charter school officials are scrambling to formulate their budget now that they finally have some numbers to work with. The district will receive $617,414 from the state. This amount will be augmented with money from the school's fund-raising organization, Options in Education.

"We have nothing," said Mr. Mosler, who is also the treasurer for the charter school. "We just recently ordered our office computer and a photocopier and a used fax machine -- and that's all we have. We have to get our program together. It definitely gives us some concrete number so we can project what we can do next."