Tentative agreement was reached yesterday between Detroit's Board of Education and 11,000 teachers on strike since Sept. 13, and a judge in Teaneck, N.J., told strikers to return to work or be fired.

Detroit Superintendent Arthur Jefferson said classes for 200,000 students could resume today. Details of the agreement were not announced.

Teachers union president John Elliott said that the union's executive board approved the pact and that he expects teachers to approve it in a vote today.

Teachers in two Pennsylvania school districts returned to classrooms, while educators in a Montgomery County, Pa., district walked off their jobs, bringing to 21 the number of teachers' strikes in the state.

In Teaneck, Superior Court Judge Sherwin Lester said: "As a last resort, any teacher not on the job by Thursday at 12:30 p.m. . . . shall be deemed to have resigned from his employment with the board."

Lester told school board attorney Malachi Kenney that, if by Thursday "the board fails to act" by dismissing the teachers, "I will act for them." Kenney said the board would balk at firing the strikers.

Lester's ruling came during the 20th day, and 10th school day, of the strike.