Mayor Ray Nagin (search) warned residents Monday to prepare for another evacuation if Tropical Storm Wilma strengthened and moved toward the hurricane-weary city.

The storm's outer edge was near theCayman Islands (search), but forecasts said it could pose a threat to the U.S. Gulf Coast this weekend.

"The people that are moving back to New Orleans should be very mobile. That's why we have not encouraged the repopulation of children nor senior citizens that are not very active," he said. "We're going to continue to monitor the storm and at a moment's notice people should be ready to evacuate."

After Hurricane Katrina (search) devastated the city Aug. 29 and Hurricane Rita (search) reflooded it weeks later, Nagin said levees had been rebuilt to at least a 10-foot barrier level. He said that could only provide protection from a hurricane with sustained winds up to 110 mph.

Nagin said about 150,000 to 200,000 people have been in New Orleans during the day, while only 60,000 to 75,000 people were believed to remain overnight.

The city had nearly half a million residents before Katrina left tens of thousands of homes uninhabitable and ruined much of the community's infrastructure.

Nagin said buses were prepared to evacuate residents if necessary.

Meanwhile, the mayor said the Army Corps of Engineers (search) had begun assisting with garbage removal and officials were working toward weekly pickups.

Nagin also said the city had identified spaces for 3,500 trailers to house city employees, hospital workers and other employees of critical businesses. He said he was looking for places for up to 20,000 trailers.

Also Monday, the first school to reopen in the French Quarter (search) began accepting students. Only about a dozen of Cathedral Academy's original 125 students have been able to return, but more than 100 from other areas have enrolled so far, including children of city police officers living aboard a cruise ship on the riverfront.

The school escaped Hurricane Katrina's devastating floodwaters, but many of its classrooms were damaged by wind and rain. As repairs continue, classes will be held in the auditorium.