A strike by Mississippi's public school teachers, the lowest-paid in the nation, threatens to spread today with expiration of a court restraining order. The order was issued Feb. 23 by Hinds County Chancellor Paul Alexander, who scheduled a hearing today in Jackson. Atty. Gen. Ed Pittman said he would not ask that the order be extended but would work on "other legal methods of keeping the schools open." The strike, which began last Monday in southern Mississippi, has spread to 28 districts thus far, affecting 87,625 students and more than 4,800 teachers.