Prince George's County Schools Superintendent John A. Murphy has placed a strong emphasis on discipline programs in his fiscal 1986 budget proposal, his first opportunity here to seek funding for his favorite programs.

The $362.5 million budget request, which was released yesterday, includes $546,000 for a countywide alternative high school -- which chronic troublemakers could attend for up to two years -- and for suspension centers in each of the county's 20 high schools. The centers would be designed to keep suspended students isolated from other students but in a school setting.

Unlike many of the surrounding jurisdictions, Prince George's does not have any alternative schools for students who repeatedly get into trouble. Disruptive students now are suspended or, if caught carrying a weapon or selling drugs, expelled.

"If youngsters are disruptive, they must be dealt with very firmly, very fairly, but very firmly," said Murphy, who is in his first year in the county post.

But, he added, "we owe that youngster . . . not to put him out on the street but to deal with him. We're not helping the youngster and we're not helping society by simply excluding that youngster."

The disciplinary programs represent the second biggest request for new budget items. The most expensive addition is $2.3 million to reduce average class size by one student.

The reduction would make the typical class 28 students and would require hiring 126 teachers, Murphy said.

The budget proposal, which must still be approved by the Board of Education and county officials, represents a $28.9 million, or 8.7 percent, increase over the fiscal 1985 budget of $333.5 million. Seventy percent of the increase is necessary to maintain current services, because of contractual obligations and inflation, Murphy said.

But county government officials have been skeptical that the schools will receive as much as Murphy wants. The superintendent said that after an increase in state funding and county revenues is taken into account, he still expects to be $15 million short of the $28.9 million increase he is seeking.

Also included in the budget are funds for increasing the number of elementary school computers, guidance counselors, library aides and new textbooks. Substitute teachers' salaries, now $37 a day, would be raised an average of 12 percent under the budget. Murphy proposed after-school day care for students whose parents work but the centers would be funded through tuition, not county money.

Murphy said he envisions the alternative high school as a setting where students who were headed toward expulsion are shifted to continue their academic programs under rigid disciplinary standards. But they will not be allowed to participate in extracurricular activities, including sports and dances.They also would be given counseling and remedial academic classes, he said. If students do not improve their behavior in two years, they will be expelled, he said.

If approved by the board, the alternative school would eventually hold about 400 students, he explained, beginning with a pilot program half that size. No site for the school has been chosen, he said.

Murphy used similar schools successfully in previous jobs in North Carolina and Illinois, he said. That experience led him to conclude that many of the students with discipline problems also have learning disabilities.

"Once students learned to read and learned long division, then their behavior started to change and they got back into the [regular] schools," he said.