A higher than expected student enrollment this year has prompted Howard County educators to revise their building plans for the 1990s to add one elementary school and to move up the completion date of another, according to a budget proposal released last week.

Superintendent Michael E. Hickey said last week that 26,630 pupils showed up for classes last month, 950 more than last year and about 135 more than had been predicted. At that growth rate, school officials now estimate enrollment increases will demand enough space to fill a 544-capacity elementary school by the fall of 1992, he said.

Officials already planned to open another elementary school in the Ellicott City area that year. But with principals in the county's northern region complaining about having to use cafeterias and closets to accommodate students, Hickey has asked for funds to finish the project by the fall of 1991.

The facility planned for 1992, which will be built at a site yet to be determined, is the fifth new elementary school officials want to add in Howard, which, like neighboring Anne Arundel and Montgomery counties, has experienced a recent surge in the number of students. They also plan to build three middle schools by 1993, when the school population is expected to exceed 33,000. In addition, school plans call for opening a fourth middle school and a new high school in the fall of 1995.

None of the schools are planned for Columbia, which has traditionally been the county's focus of growth. Along with additions to schools and building maintenance, the six-year capital improvement plan is expected to cost $104.5 million. Howard Associate Superintendent Charles I. Ecker said last week that he expects the state to pick up no more than $12.5 million of the $104.5 million capital improvement program.

The blueprint for growth promises to figure largely in the county's annual budget debates. Hickey's proposal for the fiscal year that begins in July would cost $20 million, less than $5 million of which would come from the state.

At the same time, other county departments have several big-ticket items on their wish lists for the coming year, including $7.5 million for new roads and $2.5 million for a police and fire communications system.

Last year, a committee appointed by County Executive Elizabeth Bobo recommended that the county issue no more than $25 million in new debt annually. The County Council eventually voted 3 to 2 to go over that voluntary limit when it appropriated $7.3 million for a new elementary school in Savage but indicated in a recent meeting with school board members that they would be reluctant to do so again.

"If we don't add the space, we're going to have to look at a list of alternatives, such as double shifting, increasing class sizes or year-round school," Hickey said. "None of those alternatives, I believe, would be politically popular."

Bobo said last week that while she would be unlikely to go over the spending limit this year, the county has other options, such as financing school construction on a pay-as-you-go basis. She added, however, that at this point, at least, she would be unwilling to raise taxes to pay for the building boom.

Although the budget season doesn't officially begin until January, school officials are already warning of a potential financing crunch in their operating budget as well. Hickey said that preliminary estimates show that they will need a 15 percent increase in their operating budget next year to increase teacher salaries and buy supplies for the additional students.