Just before 9 a.m., they file into large, sometimes windowless rooms.

In some cases, they punch time cards; in others, they scribble their names on a sign-in sheet.

They take their places in plastic chairs either grouped around tables or scattered haphazardly.

Some immediately pull out crossword puzzles or books. Some knit. Others hold golf-putting contests. One takes out his guitar and strums.

One day last week, another, wearing a leotard and tights, spread out on the floor and stretched before practicing ballet against a wall in a corner.

Nearby, gazing out a window, a man slowly fell asleep, his head in his hands.

It’s all in a day’s work on the city payroll.

For seven hours a day, five days a week, hundreds of Department of Education employees – who’ve been accused of wrongdoing ranging from buying a plant for a school against the principal’s wishes to inappropriately touching a student – do absolutely no work.

In an investigation inside the nine reassignment centers called “rubber rooms” where these employees are sent, The Post has learned that the number of salaried teachers sitting idly waiting for their cases to be heard has exploded to 757 this year – more than twice the number just two years ago – at a cost of about $40 million a year, based on the median teacher salary.

The city pays millions more for substitute teachers and employees to replace them and to lease rubber-room space.

Meanwhile, the 757 – paid from $42,500 to $93,400 a year – bring in lounge chairs to recline, talk on their cellphones and watch movies on portable DVD players, according to interviews with more than 50 employees.

David Pakter, 62, has been in a rubber room for a year for buying a plant for his school and giving students watches he’d made, he said.

The DOE would not discuss ongoing investigations.

Pakter, a former “teacher of the year” honored at City Hall during Rudy Giuliani’s mayoral tenure, just bought a new Jaguar with his $90,000 salary for “doing absolutely nothing.”

“It’s a present from [Schools Chancellor] Joel Klein,” Pakter said. “I want to teach, they won’t let me teach, but they’ll pay me enough to buy a car. Can someone explain this to me?”

Another rubber-room attendant said she was unaware of the reason she’d been assigned there for more than a month. Yet another, an Army reservist who spent almost 3 1/2 years in a rubber room before he retired, begged to be able to go to Iraq instead of staying in DOE Siberia.

Sam Lazarus, a social-studies teacher and union rep at Bryant HS in Long Island City, said he and his spotless 23-year teaching record were shipped off to a Queens rubber room last spring that was so “filled to the rafters” that he was forced to sit in a plastic chair outside the room.

“I think I’m the only one who actually had a grievance to get into a rubber room,” said Lazarus, who was in this “detention” for six weeks before accusations of verbally abusing a student were dropped.

Some say the teachers themselves are to blame – their union contract requires a hearing before any tenured employee can be fired.

“The reason the rubber room exists is because of worn-out and, quite frankly, irrelevant union contracts that do more to protect people’s jobs than they do to protect kids,” said Jeanne Allen, president of the Center for Education Reform, based in Washington, D.C.

Adding to that issue is the fact that the 20 arbitrators who review cases meet, on average, five times a month, or twice a month in the summer, making for a painfully slow and inefficient system.

Meanwhile, some teachers feel they’re being attacked in a “guilty until proven innocent” atmosphere in which more powerful principals can easily remove teachers who question the system and students can easily get teachers they don’t like removed by making up accusations.

The union now counsels its members to avoid becoming too involved – including even in breaking up student fights – because it could land them in a rubber room.

“Teachers are scared. The system wants to cover itself, not protect us,” said Lenny Brown, a physics teacher who landed in the rubber room over accusations that he touched a student’s breast in front of the class – a charge he vehemently denies.

The DOE admits it has become more proactive in trying to fire teachers.

“I’ve been pushing to try to charge more cases,” said DOE general counsel Michael Best. “Sometimes I’d rather be more aggressive in terms of things, in getting folks who we should be trying to terminate terminated.”

But Best says the system, governed by state law and contractual obligations, is cumbersome.

“I think everybody would like to resolve these cases more quickly,” he said.

Randi Weingarten, president of the United Federation of Teachers, says the union is not interested in protecting bad teachers and adds that she is willing to compromise to fix the system. She hopes the future holds a quicker process and a rubber-room system where teachers will at least be working.

The city used to assign teachers under review administrative duties, but that ended with the 2002 school reorganization when district-run rubber rooms were turned into larger, citywide cells.

The DOE says it’s handcuffed by a clause in the teacher contract saying rubber-room residents cannot be given “non-teaching duties,” but the UFT says administrative work is just fine.

“Right now, the system is terrible for everyone,” said Weingarten. “It’s in everyone’s interest, I think, to change it.”

Additional reporting by Susannah Cahalan and Kathianne Boniello

angela.montefinise@nypost.com