A negotiated 6 per cent salary increase for Montgomery County's teachers, coupled with heavy increases in insurance premiums and a steep rise in utility costs, have helped boost the proposed cost of operating county schools next year to $265 million.

The budget proposal released Thursday night by School Supt. Charles M. Bernardo calls for an 8.9 per cent increase over this year's $243 million budget, and the proposed $265 million total is far in excess of the County Council's suggested budget ceiling of $250 million.

In addition, the impact of the proposed budget on county finances would be magnified by the continued erosion of state and federal support for school operating costs. Bernardo projects a $27 million increase in the county funds contributed to the budget, bringing the county's share to $208,484,307.

"Costs we cannot control are responsible for this year's budget increase," Bernardo said in a statement accompanying his budget proposal.

"Many reductions have been made to partially offset these higher costs without substantially harming the quality of the educational program. But a public school system can no more absorb inflated costs without passing them along to the consumer than a business can."

Bernardo's budget proposal must be shaped by the full Board of Education into a final budget request, and this must be submitted to the County Council by March 1.

Board member Roscoe Nix said last night that "I suspect some cuts will be made in the budget" by the board, but, he added, "It's my opinion the budget doesn't have too much fat in it."

However, John Menke, president of the County Council - the group that will have the final say on the budget for the schools - said yesterday that funding the superintendent's request would require a 10 per cent increase in county residents' property tax bills.

Menke also said that, if the Council agreed to the 6 per cent wage increase, which the teachers' union and the board have negotiated, it would cost $20 million, since, he said, county employes would have to receive raises commensurate with the teachers.

Despite the expressions of disapproval from Menke and fellow Council member Dickran Hovsepian at the size of the budget, Bernardo's proposal is the first superintendent's request in 25 yearsa to propose a decrease in the number of school employes.

In response to declining enrollment, Bernardo proposed decreasing the number of school positions by 100 to a total of 12,018 - a change that, a spokesman said, would most likely be accomplished through attrition and the reshuffling of personnel.

The greatest increase in the proposed budget is the nearly $9 million increase in instructional costs, which make up 65 per cent of the school system's budget.

The next largest increase in the budget proposal is a $7 million, or 45 per cent, jump in the category that includes such items as health benefit plans and insurance costs.

The cost of health benefits jumped from $5.6 million to $10.9 million, partly because the school system has to obey a court order to pay a retired employes' benefit program $2 million that was cut from the current year's budget.

In addition, the premium costs for liability insurance for the schools more than doubled, from $115,000 to a projected $260,000 because only one insurance carrier would agree to cover board members, who may be sued individually for their official actions.

Aside from the decrease in the number of jobs, among the money-saving moves proposed by Bernardo are a decrease in the number of administration areas in the county, from six to five - estimated savings $372,000 - and the closing of an unspecified number of small schools next year.

The board expects a decline next year of about 4,600 students from the countys current school enrollment of 117,000. A trend of declining enrollment has existed for about two years.

The board closed seven elementary schools last year; it has closed five so far this year and may close two more in the next month.

Besides forcing the closure and the consolidation of the county's schools, the decline in the number of students also cuts into the revenues coming into county schools. State aid formulas, for instance, are based on enrollment - and thus state aid for operating expenses should continue its current decline.

Also, federal grants to school systems that have non tax-paying federal installations in their areas are steadily melting away. This federal impact aid to Montgomery County totalled $6.2 million in 1974, $5.6 million this year, and is projected to go down to $2.4 million next year.

Finally, the severe winter and the unexpected utility bills have meant an increase of $790,525 in the proposed budget over last year's.