Loudoun County voters could face a $10 million school bond referendum in May if the School Board approves expansion plans presented to the board last night.

The board is expected to vote on the plan early next year. If it is approved, it will be forwarded to the Board of Supervisors with a recommendation for a bond referendum at the earliest opportunity, according to Edgar B. Hatrick, the county's assistant superintendent for planning and pupil services. Hatrick said the earliest a referendum could be held is May.

The plan, drawn up by school staff members, does not include any new schools for Loudoun County, which has 13,620 students enrolled this year, an increase of about 300 from last year. Among the recommendations for expansion are a new cafeteria and eight new science labs and classrooms at Loudoun County High School. The existing cafeteria would be renovated and used for music instruction, and the school's library would be expanded at a total cost of $4.2 million.

Officials also proposed that classrooms at seven elementary schools be fitted with window air conditioners, that the schools' libraries and multipurpose rooms be air-conditioned and that their electrical systems be upgraded, all for $782,200.

In addition, the three county high schools not yet air-conditioned would get central air conditioning for $5 million.

School administrators said growth projections do not warrant additional school construction at this time, but they said that within the next year the board will probably need to begin planning another elementary school to open in 1991 in the fast-growing eastern end of the county.

More time is needed to study population trends before the county commits to building additional high schools and middle schools, the staff report said.

"Economic uncertainty has forced us to be more tentative and more conservative in our projections," Hatrick said in an interview. "But what has really caused us to be conservative is the numbers. The number of students being generated by new housing is lower than we anticipated."