Montgomery County Executive Charles W. Gilchrist, hoping to preserve the county's coveted AAA bond rating, said yesterday he plans to cut nearly $100 million from a six-year, $248 million building and school renovation program approved by the Board of Education.

The budget cuts, which Gilchrist plans to announce Jan. 2, could cancel or postpone construction of one new high school, a new vocational-technical school and several of the 11 new elementary schools requested by the school board in its capital improvements program for 1986-91. Although Gilchrist would not say which schools would be affected, most of them were slated for the upcounty area north of Rockville, which has experienced an explosive population growth in recent years.

In interviews yesterday, Gilchrist and Jacqueline H. Rogers, his chief budget adviser, stressed that specific cuts in the school construction program were still tentative. At the same time, Gilchrist called the school construction program "enormous" and said it would have to bear the brunt of a $100 million reduction he intends to make in the $601 million bonding program for school, highway and sewer construction over the next six years.

School Superintendent Wilmer S. Cody reacted to Gilchrist's plan by saying all the new schools "are needed within the time frame" of the next six years.

"Eliminating either one of the high schools will leave us with overcrowded school buildings," Cody said, adding that a new vocational center "is badly needed . . . espe- cially in the upcounty area."

The added cost of high schools, combined with new facilities and millions of dollars in renovation of older schools "is more than we can pay for," Rogers said.

She said the major snag in Montgomery's ambitious plan for new schools is a decision by state officials to reimburse the county for only 10 percent of construction costs.

The added burden on the county government's finances could threaten its AAA bond rating, a key index of Montgomery's economic health that is crucial to attracting investors and sustaining public works projects such as new highways and school facilities. AAA is the highest possible rating given by New York financial houses to bond offerings and means that the bonds are considered a safe investment and the interest the county must pay on them is lower than on bonds with lesser ratings.

"The county cannot pick up the entire freight" of the school board's construction plan, Rogers said. "They're not asking for the sky, but the full burden is more than we can bear."

Cody and Gilchrist have scheduled a private meeting today to discuss the executive's planned budget reduction. Gilchrist also plans to meet with upcounty community leaders on Saturday to discuss alternatives to a massive school construction program.

Gilchrist's cuts in the school construction program, if approved by the County Council in forthcoming budget debates, could have an impact far wider than simply causing classroom overcrowding, county officials said. They could force the school board and education planners to consider altering existing school boundaries, a politically sensitive step that could affect racial balance in older schools in the southern part of Montgomery, officials said.

Earlier this year, at Gilchrist's urging, County Council members approved $90 million for school construction and renovation for 1984-85, an allocation that included seed money for the proposed Watkins Mill High School near the huge Montgomery Village housing development and for Quince Orchard High School west of I-270 near Gaithersburg. Each new high school would cost $20 million, officials said.