The 4.9 percent figure that Schools Chancellor Joel Klein has given for each school’s budget cut seriously underestimates the severity of the cuts, especially at schools where teachers want to stay, a principal told me yesterday.

When he first got his new budget from the Department of Education, the Brooklyn principal, who asked to remain anonymous, saw a cut of almost 5 percent, as Klein had warned. That amount was significant, but he could handle it. Things looked much worse when he saw that his expenses were also rising, by nearly the same amount that was being cut. In total, he said, his effective budget is set to drop by 8.5 percent, a size that means he will probably have to let go at least one teacher.

The second surprise came when he realized what was driving the rising costs: The arguably good news that staff members at his school, from teachers to school secretaries to administrators, are sticking around. That means they have more years of experience than they have in the past — and therefore must be paid more, thanks to the salary structure in schools gives teachers and other employees, including the principal himself, more money every year they stay in the system.

Klein explained the phenomenon at a recent City Council hearing on the Department of Education’s proposed budget.In the past few years, the city’s average teacher salary has been edging upward, a result of better teacher retention, Klein said. That wasn’t a problem in the past because the system’s budget was expanding faster than the salaries were increasing. But this year, Klein said, the department’s overall budget is staying flat, turning salary increases into what is effectively a system-wide budget cut.

The same phenomenon is happening at the school level. Holding onto teachers helps schools maintain stability, but it also means their budgets are locked into annual increases. The recession could inflate costs even more, since fewer teachers are expected to leave their jobs in the tough economy. That means teachers who in other years would have exited the system and been replaced by younger, lower-paid teachers are instead staying in the system and racking up salary increases.

The principal I spoke to said his 8.5 percent budget cut is going to force some tough choices. Even before he realized that was his number, he considered cutting down on payments for teachers who work extra hours, buying fewer supplies, and forgoing substitutes when teachers are absent. Now, he says he’ll talk to teachers at the school to brainstorm ideas about what to cut before the June 18 deadline to submit his budget. (What is your school cutting?)

Yet even as he struggles with the cuts, the principal said he is glad things aren’t worse, which they could have been. “Thank God — and Obama’s election — for ARRA,” the principal said, referring to the American Recovery and Reinvestment Act, the official name for the federal stimulus package. Without the hundreds of thousands of dollars that the stimulus put back into his school’s budget, the school would have been down more than 15 percent of its budget, he said.

“Could we function without it?” the principal asked. “Probably not.”