Some of New York City's best students walked back into school on Tuesday for the first time since they looked out their classroom windows to see the nearby World Trade Center come under attack.

"I'm not looking forward to psych class," said Alice Chan, a 15-year-old junior who was in that class on the eighth floor of Stuyvesant High School when the trade center was hit. "I'll just try not to look out the windows."

After the Sept. 11 terror attacks, the school was turned into a triage center until it became clear there were few survivors to treat. It then had to be thoroughly tested for asbestos and cleaned. The students had spent a month at Brooklyn Tech, another of the city's top-ranked schools.

The reopening of Stuyvesant came as National Guard troops took their posts at bridges, tunnels and train hubs across the city. Officials said New Yorkers — and all Americans — should get used to living in an atmosphere of heightened security.

As the students walked back into Stuyvesant, they could see a transformed neighborhood: Heavy police patrols, National Guard troops in camouflage gear, dump trucks filled with debris motoring past on West Street — and the gap in the skyline where the Trade Center had been.

"It's terrible," said junior William Barnard, 16. "When I first got here I looked around. It's just not there."

Also returning was Jessica Schumer, accompanied by her father, Sen. Charles Schumer. Asked if he was worried, the senator said, "They've done all the testing. The school is safe."

Police Commissioner Bernard Kerik said New Yorkers should continue their routines even as they get used to tougher security.

"This is something they should get accustomed to, not only in New York City but probably across the country," he said on NBC's Today show Tuesday.

The National Guard joined police in patrolling Penn Station on Tuesday. Gov. George Pataki said the move, similar to one already taken at area airports, was not prompted by any specific incidents.

On Monday, guardsmen patrolled the Brooklyn Bridge, including its pedestrian walkways. Armed officers were posted outside city, state and federal buildings.

New York City has been on its highest level of security alert since the attacks. Condition "Omega" calls for enhanced security around sensitive areas, such as government buildings, houses of worship, landmarks, tunnels, bridges and subways.

A poll released Tuesday found New Yorkers' fears of another terrorist attack in the city have not diminished. The Marist Institute for Public Opinion found that 73 percent of New Yorkers are either "very worried" or "worried" about another major terrorist attack, which is up three percent from the same poll taken in mid-September. The poll of 1,275 New Yorkers, conducted Oct. 2-4, had a margin of error of 3 percentage points.

Meanwhile, huge cranes continued to dip into a mountain of smoldering rubble at the trade center disaster site, lifting several-ton sections of what were once support beams. The movement of debris released 100-foot-wide plumes of smoke from fires within the ruins. At least one body, draped in an American flag, was seen being pulled from the ruins.

The number of people reported missing dropped Monday to 4,815. There have been 417 confirmed deaths, including 366 victims who have been identified.

The mayor, remembering the 343 Fire Department members and 23 police officers lost in the attacks, said: "I know if they were here, they'd say, 'Be brave. ... We died to protect freedom."'