BATTERY PARK CITY — When Stuyvesant High School's 3,000 students returned to their Battery Park City classrooms on Oct. 9, 2001, they thought the worst was over.

Less than a month earlier, the 14-to-18-year-olds had fled their school, some in tears and some numbly silent, as the World Trade Center collapsed just three blocks behind them.

The students at one of the city's most selective high schools were ready to get back to their routine of coursework and college applications, and ready to reassemble the pieces of their pre-9/11 academic and social lives.

But from their first breath of smoky, dusty air they took as they emerged from the subway that October morning, many of the students feared that they had returned to lower Manhattan too soon.

"We could see the fire burning [at Ground Zero] on our way to school," said Lila Nordstrom, who was a senior on 9/11.

"It was very clear it was not safe."

Nordstrom, now 27 and living in California, later developed chronic bronchitis and severe acid reflux that she believes is connected to her daily exposure to 9/11 toxins at Stuyvesant in the fall and winter of 2001.

And she is not alone. Nordstrom, who started an advocacy group called StuyHealth, has spoken to dozens of her former classmates who have illnesses that may be connected to 9/11.

Some of them are receiving treatment at Bellevue Hospital's World Trade Center Environmental Health Center, where staff confirmed they are seeing former Stuyvesant students but would not say how many.

Amit Friedlander, Stuyvesant's senior class president on 9/11, remembers feeling uneasy about the air he was breathing as he walked to and from Stuyvesant each day in the fall of 2001.

But like many of his classmates, Friedlander was not willing to transfer to another high school, after working so hard to earn his Stuyvesant diploma.

"I figured the best thing to do is not to think about it," Friedlander said of the potential danger.

"Once you make a decision to do something risky, you can't think about it."

So Friedlander put the dust and smoke out of his mind — until the summer of 2006, when, after months of looking so tired that his mother asked if he was on drugs, Friedlander was diagnosed with Hodgkin's lymphoma.

The doctors told Friedlander they couldn't say for certain whether his illness was caused by 9/11.

But Friedlander has no doubt the two are connected.

"If we hadn't been sent back so soon, I wouldn't have cancer," said Friedlander, now 27, who battled the disease and has been healthy for the past four years.

"That made me really mad."

Even 10 years after 9/11, no one knows exactly what toxins the students at Stuyvesant High School were breathing and how it could affect them.

Air monitoring at the building showed occasional elevated levels of lead dust and particulate matter. Starting on the morning of Sept. 11 until shortly before the students returned, Stuyvesant had been used as a command center, with hundreds of officials walking back and forth to Ground Zero and tracking dust into the school on their boots.

And every day in the fall of 2001, as the students walked along Chambers Street, trucks filled with smoking World Trade Center debris rumbled past, depositing the rubble in an enormous pile on a barge just north of the school.

Stuyvesant principal Stanley Teitel, who was principal in 2001 and remains at the school this year, declined to comment. The Department of Education did not immediately respond to calls for comment.

But according to those enrolled in the school that year, students began complaining of headaches, sore throats and nosebleeds. Several teachers left out of concern for their health. Former Schools Chancellor Harold O. Levy moved his offices to Stuyvesant to prove that the air was safe, but then he returned to Brooklyn just a few days later, according to reports.

"In spite of all the assurances from all levels of government, we very quickly realized they were lying to us and it was not safe," said Marilena Christodoulou, who was the president of the Stuyvesant Parents' Association on 9/11.

The former head of the Environmental Protection Agency, Christine Todd Whitman, has come under attack for her official statement in the days after 9/11 that the air around Ground Zero was safe to breathe, despite the mounting evidence to the contrary.

Christodoulou led the advocacy for a more thorough cleanup, particularly of the school's sprawling ventilation system and the auditorium, where porous materials like the carpet and the stage curtain had not been replaced after the towers fell.

In November 2001, after students had been back for a month, Nordstrom remembers a sudden directive not to drink out of the water fountains anymore.

"It didn't add up to a safe environment," she said.

But Nordstrom, like many of the students, did not feel that she had any choice in the matter.

"The city made the decision for us," Nordstrom said. "We were kids. We couldn’t not go to school."

After the Parents' Association threatened a lawsuit in the spring of 2002, the city finally agreed to shut Stuyvesant down that summer and give it the comprehensive cleaning advocates said it should have gotten before students returned more than six months earlier.

The cleaning came too late for Nordstrom, whose existing athsma — which had been under control before 9/11 — flared up again after the attacks, along with increasing respiratory and gastrointestinal problems.

Now, Nordstrom said she is particularly concerned when she hears about the illnesses first responders and recovery workers are contracting, including cancers and life-threatening lung ailments.

"First responders are getting the health problems now that we're going to get in 20 years," Nordstrom said.

"I'm scared for myself…and I'm angry."