Registration for voting in the up- coming municipal election closes tomorrow, less than three weeks before voters will head to the polls to elect a new city council and school committee.

City officials, who predict a repeat of the usual last-minute crush of registrants, said yesterday the Election Commission office on Green St. in Central Square will stay open until 10 p.m. tomorrow. It will be open from 8:30 a.m. to 4:30 p.m. and from 6 to 7 p.m. today.

Student registrants number between 700 and 800, and could play a large role in the election, a student, Guy Molyneux, said yesterday. Molyneux, an organizer for the Democratic Socialist Organizing Committee (DSOC) helped distribute leaflets last weekend backing the candidacy of tenant advocate and council incumbent David Sullivan.

And several referenda issues--including a call for an end to U.S. involvement in El Salvador--may also help draw student voters to the polls, officials of the MIT Committee on Central America, said yesterday.

The committee will concentrate its organizing efforts elsewhere, but press for high student turnouts in the last days before the elections, Clifford Truesdell, a committee leader, said last week.

Registration is open to all residents of the city 18 years of age and older. The last municipal election, when student voters overwhelmingly supported Sullivan, was the first big turnout of students.