D.C. Schools Superintendent Floretta D. McKenzie has proposed a $42 million budget increase for the coming school year, most of it for increases in teachers salaries, improvements in junior and senior high schools and special education programs.

In the proposed budget, which was submitted to the school board last month but not made public until yesterday, McKenzie asked for a total of $402.8 million for fiscal year 1987, which begins Oct. 1, 1986. This is 11 percent more than the current school budget.

About $12 million, or 30 percent of the increase, would be used to give teachers, counselors and other school personnel the 4 percent pay raise that was approved by the school board this year. An additional $4 million would be used to pay to promote hundreds of employes.

McKenzie also proposed spending $4 million to hire 62 new mathematics and English teachers, in order to reduce the number of overcrowded classes in the city's 40 junior and senior high schools, plus 25 new counselors to reduce the counselor-to-student ratio that now stands at more than 400-to-1 in some schools.

An additional $4 million would be used to hire 110 special education teachers and aides, in an effort to "mainstream" learning-disabled students.

McKenzie could not be reached for comment.

James Brown, associate superintendent of the office of management services, which drafted the budget request, said: "Aside from the salary increases and promotions, the superintendent's objective in this budget request is to make a giant leap in progress on the secondary school level.

"In recent years, we have been able to make considerable improvements in the elementary schools, but our older students have continued to slip behind. In the 1987 school year, we plan to make a big leap forward in upgrading the quality of instruction. We expect to make substantial gains in that area."

Although elementary students' grades have been improving, junior and senior high school students generally continue to score below the national norms on standardized mathematics, reading and science tests, according to school reports.

The rest of the proposed budget increase would be spent on programs that range from installing doors and alarm systems in school buildings and establishing more all-day, prekindergarten classes, to buying new computer software.

Public hearings on the proposed budget began earlier this week. After the school board acts on it, the budget will be sent to the mayor to be incorporated in the 1987 budget that he will present to the City Council for approval.