Montgomery County's 7,500 teachers and 5,000 school custodial and clerical employees reached impasses in their contract negotiations with the county over the holidays, thus forcing the contract disputes into the hands of outside medicators.

It is the third contract in a row forced into mediation for both groups. Even if the county school board approves the mediators' recommendations - which it didn't last year - any raises must still be approved by the county council, which adopts school budgets. The contracts do not expire until June 30, after the school year has ended, but negotiations begin nine months early because of the complicated school budget process.

Starting salaries for Montgomery County teachers this past year were the lowest in the Washington area, at $9,415, and top teacher salaries of $19,865 were next to the lowest. Supporting school employees - teacher aides, cafeteria workers, clerical and custodial employes - ranked roughly in the middle for area school salaries.

Both teachers and other school employees received 3 per cent wage hikes last year, a year in which many school systems here raised salaries by 5,6 and 7 per cent. The cost of living rose by 7 per cent in 1975 and by about 5 per cent in 1976, according to preliminary figures.

The Montgomery County Education Association, which represents the teachers, has already asked for raises of 13-14 per cent in the new contract, and Vincent Foo, president of the Montgomery County Council of Supporting Service Employees - which represents most non-teaching school employees - has indicated his union will want at least "the 5-6 per cent increases" now being negotiated in other area school systems.

While salaries are the major issues this year, many other employee benefits, duties and working conditions are also in dispute, and will be studied by mediators. Details about the contract proposals are not made public until the mediator makes his recommendations. Kenneth Muir, spokesman for county schools, this week said all he could say about the coming contracts at this point was that every 1 per cent incrase in salaries will mean about $1.8 million in additional costs to the county.

Muir said current salary levels make county teachers about the lowest paid in the Washington area and non-teaching employees about in the middle of area salary scales. Arlington teachers are presently the highest paid locally, with starting salaries of $10,547. Alexandria offers the highest pay to top teachers - those with PhD degrees - of $23,312, with Montgomery County's $19,865 ranking only above the $19,639 paid to such teachers in Anne Arundel County.

Teacher assistants or aides in Montgomery County now are paid $4.04 to $5.17 an hour, higher than the $3.83-$4.99 an hour aides are paid in the District, but lower than Prince George's County assistants, who get $4.38 to $5.79 an hour. Montgomery County $5.79 an hour. Montgomery County secretarial salaries range from $9,089 to $11,752, also roughly in the middle salary range paid to school and government secretaries locally.

Montgomery's non-teaching employees "used to be among the best paid in the Washington area," said Foo, "but we've been falling behind in the past three years, getting only 3 per cent raises while other jurisdictions have been giving 5,6 and 7 per cent and now are again negotiating 5 and 6 per cent raises." Foo said the outside mediator last year recommended a 6 per cent boost for Montgomery's non-teaching school employees but said "the teachers agreed to 3 per cent and it pulled the rug right out from under us."

Foo said he was also concerned that "going through this procedure, with a mediator having to be called in for every contract, has become a routine. I think there could have been a lot more effort made to reach an agreement" during three months of bargaining that preceded the "impasse" announced last week.

Under state law, when the two sides jointly declare an impasse in negotiations, state school superintendent David W. Hornbeck declares an impasse and a mediator, selected from the American Arbitration Association, is called in. The mediation process is expected to be completed sometimes this month.