Montgomery County School Superintendent Wilmer S. Cody last night proposed raising the school system's operating budget 8.4 percent next year to $473.8 million, an increase he said is needed in part to cope with the second wave of baby boomers flooding into the public schools.

A portion of the additional $36.5 million Cody has requested would go toward hiring 120 new teachers for the 2,610 additional students expected to enter the elementary schools in 1987.

Those new staff positions, together with previously negotiated teacher pay raises of 3.7 percent and other cost increases that would maintain current services, account for $27 million of the proposed increase.

The other $9.5 million would be used for improvements in the school system, among them hiring additional teachers and aides to reduce class size at all levels, adding elementary school guidance counselors, offering more all-day kindergarten classes, and adding extra staff to work with handicapped and foreign students.

At an afternoon news conference, Cody called his budget proposal "reasonable" in that it includes "a few modest improvements."

"We are at a time when over the next four and five years we have an opportunity to consider some broad-based improvements for students in Montgomery County," Cody said.

The proposed budget provides for 477 new staff positions, including 196 additional teachers. Of the teachers, 120 will be needed to handle the growing elementary school enrollments, and 76 more teachers and 10 aides would be hired to help reduce class size systemwide.

According to figures released yesterday, enrollment in 1987 is expected to reach 95,635, an increase of 1,563 over this year's enrollment. Although 2,610 new elementary students are expected in 1987, that number is partially offset by an expected decline of 1,175 secondary school students.

This year, total enrollment grew by 1,381 students, with most of the growth occurring in the rapidly developing Germantown-Gaithersburg area.

Cody said one of his priorities is lowering class size overall at the elementary school level and reducing the number of oversized classes at the junior, intermediate and high school levels.

Recently, a survey of 6,700 parents by the Montgomery County Council of Parent-Teacher Associations showed that they were primarily concerned with lowering class size.

The average class size in county elementary schools is 25 students per class, but with the extra teachers Cody has proposed, that number would be lowered to about 23.5 students per class.

At the junior high school, intermediate and high school levels, about 500 classes are over the countywide accepted average of 32 students or less. Cody said he hopes to cut that number in half in 1987.

The proposed budget will be the subject of several public hearings before it is adopted by the Board of Education on Feb. 6. It will be forwarded to the County Council on March 3.

County Executive Charles W. Gilchrist will recommend his own school budget to the council. Already, the county's Office of Management and Budget has released a preliminary 1987 school budget proposal of $465.3 million, or about $8 million less than Cody has proposed.

Of the discrepancy between the two budgets, Cody said: "We'll go through the same process we go through each year and will try, as rationally as we can and with justification, to make the case as to why our budget should be . . . $473 million."

Among the items included in Cody's budget:

*$300,813 to offer all-day kindergarten classes to 1,025 additional students. Currently, there are 1,600 five-year-olds in all-day kindergarten. Cody said his goal is to offer all-day kindergarten to all students in the county.

*$418,418 to fund 11.5 more guidance counseling jobs for the elementary schools.

*$2.3 million for hiring extra teachers to work with children who need special help but do not belong in learning-disabled classes, and to improve math and science programs as well as those for the gifted and talented. Some of the funds would be used to hire more aides to oversee computer labs at all class levels.

*$548,712 to hire staff to work with handicapped students, foreign students and special education students.