Prince George's County School Superintendent John A. Murphy proposed yesterday a $381.3 million education budget for fiscal 1987, endorsing a new energy tax on homes and businesses to help finance what he called "a very modest budget" in a county where resources look "rather dim."

The budget proposal, which does not account for salary increases now under negotiation, represents a 6.5 percent increase over this year's budget. It includes $12.9 million for the second phase of a magnet school plan aimed at desegregation and $2.5 million for 122 new teaching positions to reduce average class size by one pupil from 28 -- the highest in the state -- to 27.

"It's a very modest step," Murphy said of the budget proposal, which still must go before the Board of Education and County Council. "The resource picture is rather dim in Prince George's County."

The budget exceeds the $380.7 million ceiling proposed by County Executive Parris Glendening, but Glendening, citing "momentum" and new enthusiasm to improve the county's school system, said the county will come up with funds close to what Murphy is asking.

The budget also includes $600,000 for an alternative high school for disruptive students, a proposal submitted last year by Murphy but withdrawn when concerns were raised that a disproportionate number of black students would be assigned to the school.

Murphy pointed out that his budget proposal was less in dollars and per-pupil expenditures than that of either Montgomery or Fairfax county, which this week released proposed school budgets of $473 million and $589 million respectively.

An energy tax, in place in several other Maryland counties, has been recommended before for Prince George's. Glendening unsuccessfully proposed an energy tax on businesses to the General Assembly last session, and county legislators are drafting a bill this year that proposes a 3 percent levy on home and commercial energy consumption. That tax would, according to Murphy, cost the average household about $36 and raise about $15 million annually.

Glendening said his office was studying several ways to bring in new money for the schools, "ranging from increased state aid to revenue sources to significant internal adjustments." He would not say precisely what new taxes or adjustments, if any, are under study.

He was noncommittal on a new energy tax, but said he was optimistic it could get a more favorable reception this session than last. "There is probably more unity and enthusiasm for funding education than anything I have seen in a good many years," he said.

Del. Charles J. Ryan, who heads the county's House delegation, said an energy tax "won't receive the curt treatment it did last time" if state funds available for the magnet school program are insufficient.

Legislators say they believe Gov. Harry Hughes' budget will include up to $8 million of the $12.9 million proposed for magnet schools.

Paul Pinsky, president of the county teachers union, applauded some of the goals in Murphy's budget, such as smaller classes and more school library personnel, but added, "We have to remain suspect" about what salary increase teachers will receive. "What we hear from the [negotiating] team is extremely low."

School officials say they are offering incremental teacher raises that, over the course of the school year, would average 3 to 4 percent. With the accumulated raises, a typical teacher's salary would be 9 percent more by the end of the next school year than it is this year, according to the officials' calculations.

Pinsky said the teachers are asking for starting salaries of $20,000, compared with the present $15,738, the lowest in the Washington area. Murphy said raising teacher salaries is his top priority, followed by lowering class size.

The $12.9 million set aside to finance magnet schools is about $4 million more than the estimated cost for fiscal 1986, which ends June 30. The larger figure would maintain the existing dozen magnet schools and establish new ones, designed to improve integration by drawing students out of racially homogenous neighborhoods. It would also pay for all-day kindergarten at 10 "compensatory educational" schools, which are receiving extra resources because officials say their locations make them impossible to desegregate through busing.