Facing what officials called a nearimpossible deadline, the District of Columbia Board of Elections and Ethics approved a sharply expedited timetable yesterday for candidates wanting to run May 1 for a vacant seat on the school board.

Instead of 44 days to collect signatures on petitions, would-be candidates will have only 10 days, possibly starting as early as tomorrow. But if the printer cannot produce the petitions that fast, the starting date will be set back to Tuesday.

However, under emergency rules adopted yesterday, only 250 valid signatures must be collected to qualify for a place on the ballot instead of the usual 1,000 needed for an at-large race.

A special election will be held May 1, chiefly to fill two vacancies on the City Council. On Tuesday night, the council voted to add the school board vacancy to the ballots, and Mayor Marion Barry's office said he expects to sign the legislation into law today.

Lacking a special election, the school board would have been required to choose someone to fill the vacancy, created by last November's election of Betty Ann Kane to the City Council.

Delores M. Woods, deputy elections administrator, said she expects between 20 and 25 potential candidates to take out petitions for the school board race.

Without the expedited timetable, Woods said the board would be "pushed to the wall" to get ballots printed in time. She said only about two weeks will be available between the final approval of the ballot's contents and April 16, the day absentee and sample ballots must, by law, be available.