The start of the school year marked the end of careers for a virtually unprecedented number of local teachers, depriving schools of their most experienced staff.

A staggering 477 teachers retired this September, compared to last year’s 268, according to the city’s Teachers’ Retirement System. And that’s on top of another 3,868 who left over the summer. The September spike is the largest since 1991, when a special incentive was offered.

The United Federation of Teachers was quick to blame the Bloomberg administration’s recent school reforms for the walkout. “It’s a clear indication that teachers feel that what’s being asked of them is unaccomplishable,” UFT president Randi Weingarten said, though she had yet to see the month’s figures. “They’re demoralized.”

Several teachers agreed. “If I could retire, I would,” said one 19-year veteran. When she started teaching years ago on the Lower East Side, the school was empty by 3:10 p.m. sharp. Now, she says, with the amount of paperwork and testing required, many teachers stay until 8 p.m. or come in on weekends.

She also considers this year’s changes in math and reading curriculum confusing and hard to implement. For new teachers, she said, it’s even worse. “You’re left there, the door closes, you hear the theme music from JAWS,” she said. “It’s like Nightmare on Sesame Street.”

Another long-term educator chose to retire this summer rather than compete for scraps when citywide restructuring eliminated her job. When she went to the retirement office, she said, “There were people out in the hall. There were so many people there.”

The retirement boom could simply reflect a demographic shift, explained Henry Levin, professor of economics and education at Columbia University’s Teachers College. “Schools really expanded in the `60s,” he said, “A lot of people came in during that period so a lot of those people are retiring now.” The average age of teachers is 49, according to the UFT, and 23,000 are eligible to retire this year, nearly a quarter of the workforce.

Although neither the Teachers’ Retirement System nor the UFT tracks the ages of retirees, most observers say the number is dropping. Retirement packages make it hard to resist jumping ship; given the way pensions are taxed, some teachers may actually take home more money after they quit.

Regardless of the cause, the retirement boom will no doubt exacerbate hiring problems at city schools. At one Brooklyn high school, for example, math and science students were greeted by substitutes this September while school administrators scrambled to fill four vacant slots.

The Department of Education did not return repeated calls for more information. That didn’t surprise Levin, who suspects that what seems like a teacher shortage may actually be more of a logistical problem, one that keeps qualified teachers waiting despite open jobs. “If we had that information, we’d have a better picture, but we just don’t,” he said. “The system isn’t terribly transparent.”