Montgomery County School Superintendent Harry Pitt said yesterday that the school system faces a $2.2 million deficit this year that necessitates a hiring freeze.

Pitt told the county's school board that, for the rest of the school year, teachers in middle and high schools who leave will be replaced with substitute teachers.

To absorb the shortfall in its $703 million budget, the school system also will leave vacant 75 jobs in central and regional offices, temporarily freeze 85 jobs in schools, restrict consultants and overtime pay and not buy any equipment.

The deficit has materialized, Pitt said, because of unexpected expenses for disabled students and fuel and because 1,000 more students turned up this fall than officials had planned for. "We are not thrilled with the things we are doing," Pitt told board members, "but we recognize we . . . have a serious problem."

Mindful of the school system's deteriorating financial health, the school board expressed no objection to Pitt's actions and decided to take a rapid, intense look at how it can save money.

The board dispatched auditors and the school system's research staff to study ways the schools could be run more efficiently. It also decided to form a citizen committee to study the issue.

The critique, to be finished within two months, appears intended to ward off what is likely to be strong financial pressure from the county during the next several months.

The 1991-92 budget that Pitt is due to propose early next month will require a juggling act: Enrollment is expected to grow by 4,000 students next year as the county faces severe monetary strains.

County Executive Neal Potter has warned that Montgomery faces a budget deficit next year of $100 million to $185 million.

He has said that schools, which account for nearly half the county's expenditures, will have to absorb part of the reductions, including possible changes in labor contracts and class sizes.

Also yesterday, the board elected its senior member, Blair Ewing, president. He has been on the board for 12 years.

Ewing's election marks a major power shift on the seven-member board. In recent years Ewing, of Silver Spring, has been an outspoken minority voice on the board. He broadened his influence by supporting the four candidates -- all novices to electoral politics -- who won seats last month.

He was the only member to oppose Pitt's 1987 selection as superintendent.