Two weeks after Mayor Bloomberg announced a plan to to replace half of all teachers at 33 struggling schools, efforts are underway to soften the threat.

Department of Education officials said today that the city is exploring the option of replacing fewer teachers at the schools under an allowance included in federal guidelines for the school improvement strategy known as “turnaround.”

The turnaround process, which Bloomberg announced two weeks ago to sidestep a requirement of other school improvement strategies to negotiate new teacher evaluations with the teachers union, mandates that 50 percent of teachers be replaced. But the U.S. Department of Education makes special allowances for some teachers who have been hired in the last two years.

Now the city is looking to take advantage of that flexibility when it files formal turnaround applications with the state next month.

The catch is that not every teacher hired in the last two years is automatically eligible for the exemption.The federal guidelines make an allowance only for teachers who were selected “according to locally adopted competencies as part of a school reform effort” headed by a principal handpicked to lead it. That means, according to the guidelines, the teachers should have been screened for an ability to “be effective in a turnaround situation.”

It’s not clear how many of the roughly 3,400 teachers at the 33 schools would fall into this category. As recently as Monday, Chancellor Dennis Walcott told state legislators that there would be “possibly up to 1,500, 1,700 teachers” cut loose from the schools.

But there are some clues. Twenty-seven of the 33 schools were already receiving federal funds to undergo the “restart” or “transformation” processes, and many of them received new principals in the process. The city could make the case that the federal regulations would allow any teachers hired since those processes began to stay on, although it could be a tough sell in instances where the school was continuing its regular efforts to replace faculty members who retired, resigned, or moved to other schools.

The argument would be clearest for the dozens of “master” and “turnaround” teachers brought on at the 33 schools. Their salaries were being paid for using the federal School Improvement Grants — they are the only teachers for whom that is true — and their duties were aligned with the grant program’s requirements.

City officials could not immediately identify just how many teachers have been brought on at the schools in the last two years. But the number is unlikely to cut too far into the portion of teachers required to be replaced. That’s because most of the schools have relatively stable staffs, and budget cuts in recent years have required many schools to cut their teaching rosters, not add to them.

One challenge to the move could be how to reconcile the federal requirements for restaffing with the ones contained in the city’s contract with the teachers union. In order to invoke a clause of the contract known as 18-D to restaff the schools, the city must technically close and reopen all of the schools. Because the schools will technically be new, it would be hard for the city to argue that any of the teachers had already been on staff as part of an existing reform effort.

Plus, 18-D requires the new school to hire from the old staff in order of seniority, as long as teachers meet certain qualifications. The turnaround exemption could position some teachers to leapfrog over more senior colleagues in the rehiring process.

Walcott told legislators earlier this week that the city would submit its turnaround applications to the state within weeks. State Education Commissioner John King is responsible for assessing the applications in accordance with the federal guidelines. He has called the general contours of the city’s turnaround plan “approvable.”