Attorneys in the 1974 Boston school desegregation case are seeking a court order to block a new student assignment plan that they fear could lead to a further exodus of white students from inner-city schools. The lawyers, Thomas Atkins and Robert Pressman, filed a motion with U.S. District Judge W. Arthur Garrity Jr., asking him to intervene to stop the student assignment plan. “As this plan is now drafted, it will result in rapid resegregation of the schools in minority neighborhoods,” Atkins said. Officials devised the plan in a bid to end 15 years of court-ordered busing. It would divide Boston into three zones and allow parents to choose a school within their zone--provided student populations remain within racial balance.