Prince George's County School Superintendent John A. Murphy said yesterday that preliminary projections show that the schools will need as much as $29 million in additional funds next year to maintain the current level of operation and to institute new projects he will propose, including an alternative high school for disruptive students.

Murphy acknowledged that his budget, which is expected to go before the Board of Education next week, faces an uncertain fate, given the county's financial strictures and the battles that have been waged over school budgets in recent years. If it contains the $29 million in additional funds, the fiscal 1986 budget will total $362.5 million, an 8.7 percent increase over the current budget of $333.5 million.

Murphy said the bulk of his requested budget increase -- $25 million -- is necessary "to pretty much keep on doing what we're doing . . . . No requests are coming in for major new items." He said the increase was based on inflation and contractual obligations.

County Executive Parris N. Glendening echoed the concern about how Murphy's budget would fare. "I'm philosophically supportive of making the kind of changes they're looking for," Glendening said. But he added that unless new revenue is found, county agencies will be working with "maintenance only" budgets and funds for Murphy's proposed increase will be unavailable.

"I have no idea where that kind of revenue would come from," said County Council Chairman William B. Amonett. "It would be a surprise if we had that much increase in revenue to deal with the whole county."

In addition to the alternative high school, Murphy said he will ask for funds to reduce average class size by one pupil, add computer equipment to elementary schools, hire new guidance counselors and library aides and beef up the supply of textbooks and library books.

The county's financial bind was eased last month when voters modified TRIM, the property tax revenue cap, but that change will raise only $7.3 million. Glendening said last week that more money will be necessary to reduce teacher-student ratios and make other educational and public safety improvements. He announced at the time that he was seeking authority to impose new utility and parking taxes.

The proposed alternative high school would be designed to help the "most severe disruptive students who are on the verge of expulsion," Murphy said. The aim would be to modify the students' behavior so they could return to their regular schools.

The reduction in class size would bring the average to 28 students per class, Murphy said, which is still three students over his ultimate goal of 25 per class.

Funds for teacher salaries will increase 7 percent in the 1985-86 school year under a two-year contract settlement negotiated last year. The funds to cover that increase are included in the superintendent's budget request.

The alternative school plan must still be approved by the school board. "I think that's something we're going to take a long, hard look at," said vice chairman Sarah Johnson.