The auditorium at P.S. 195 in Manhattan Beach was flooded last Wednesday. Today, the school opened its doors to students and Mayor Bloomberg and Chancellor Walcott plan to visit and welcome them back.

As more than 90 percent of city schoolchildren head to school today for their first day back after Hurricane Sandy, some with extra sweaters to ward off cold, Department of Education officials will have their sights set on the 102 schools that still cannot reopen.

The number of school buildings unable to accommodate students fluctuated over the weekend, but by Sunday night, department officials determined that 57 schools were so damaged that they must be relocated and 29 schools still lacked power, down from nearly 200 at the beginning of the weekend. Another 16 schools are housed in eight buildings that have for the last week been used as shelters for New Yorkers displaced from homes and hospitals by the storm.

The roughly 73,000 students who attend the schools are expected to return to classes on Wednesday, after the entire city takes another break for Election Day on Tuesday, when many schools will function as polling centers.

In the next two days, officials aim for power to be restored to schools that lack it, shelters  closed and cleaned, and damaged schools shoehorned into other locations. But Mayor Bloomberg said the transition back to school — coming after students and teachers alike have had their homes and neighborhoods disruption — would likely be rocky.

“We just can’t predict who’s going to show up where … and we’re obviously going to have problems,” Bloomberg said during a news conference on Sunday. “We’ll just have to bear it, but we’ll have a day between the first day and the second day of school – namely Tuesday – and we’re going to use that day to straighten things out to the best of our ability.”

The sudden relocation of 57 schools whose buildings suffered flooding, oil spills, and fires as a result of the storm has posed the most daunting logistical challenge. Some schools are moving miles away from their original locations, and others are being divided over two or three different sites, according to the department’s plans, which changed over the weekend with conditions on the ground.

“We don’t have a very large group of empty seats, but we think … by moving things around in these schools we’re able to accommodate everyone,” Bloomberg said on Sunday.

In one extreme example, John Dewey High School, which suffered an electrical fire Tuesday morning, is sending its ninth- and 10th-graders to Sheepshead Bay High School, 11th-graders to James Madison High School, and 12th-graders to the Lafayette High School campus. While Lafayette is very close to Dewey, the other two schools are about three miles away — and more than a mile away from each other — in an arrangement that some teachers said would make it impossible to continue with students’ existing class schedules.

Other schools are moving part and parcel to another building, some as far as eight miles away. But their plans are no less complex. Principals in buildings that are getting surprise co-locations are meeting tomorrow with their new neighbors to devise space-sharing schedules and strategies, while teachers in the relocated schools will use the day to set up their new classrooms.

Letting families know that their children would have one more day off was a steep challenge for the department. More than 1 million “robocalls” had gone out to families by Sunday, officials said, and the city placed full-page ads in multiple major newspapers today announcing the changes.

But Bloomberg said on Sunday that he expected some families to make their way to school this morning, only to find it dark and shuttered.

“I’m sure we’re going to miss some people,” he said. “That’s just the reality of doing something we have to do quickly.”

He added, “It is complex and people are going to make mistakes. People are going to be misinformed. We know that.”

Some families could be misinformed in the other direction. Students at some schools where power was restored over the weekend got calls telling them that they would not have classes today even though their schools are open for business. At Harvest Collegiate High School and the Beacon School, both in Manhattan, school officials reached out to students to let them know they should attend today after learning that some had been told to stay home.

Bloomberg also emphasized that reengineering bus routes for the tens of thousands of students who will be relocated would likely not come without problems, and that downed trees and many news drivers could complicate routes even for students whose schools emerged unscathed. As temperatures dipped near freezing, he warned that some students could be left out in the cold.

“I’m sure there’s going to be cases where just the driver made a mistake on the route and we didn’t get to everybody,” Bloomberg said. “We’re trying to do our best, and I want to support them and give them all the tools that we possibly can, knowing that it’s not going to be perfect.”

Even in schools that are reopening on schedule, conditions might not be optimal. Three dozen schools are opening with power but no assurance of heat, and Bloomberg said on Sunday that parents should make sure to send their children off today with “extra sweaters.”

But for the most part, the city is viewing the return to school as an important step in the storm recovery process. P.S. 195 in Manhattan Beach, Brooklyn, is located just a block off the water and suffered severe flooding, a broken boiler system, and loss of power during the storm; last week, a watermark stretched across its front door while mansions a block away had their facades completely ripped away. This afternoon, Bloomberg and Chancellor Dennis Walcott will visit the school to welcome students back to class.