After two days of defending a new policy that changed the class ranks of tens of thousands of students at public high schools, a mayoral aide said yesterday that the Department of Education will inflate students’ grades in Advance Placement courses only going forward, not retroactively.

The New York Sun reported Monday that the department had quietly added a 10% bonus to all grades earned in AP classes as an incentive for students to take the rigorous courses. Rather than announcing the policy and imposing it for future coursework, the department used a new central computer system to change the grades of all current seniors who had taken AP courses in the past four years.

Yesterday, the city’s deputy mayor for policy, Dennis Walcott, disclosed that the administration was reversing course. The 10% AP bonuses will be handed out only in the future – not to students who have taken the standardized, college-level courses in the past few years.

“It shouldn’t impact students retroactively,” Mr. Walcott said in a telephone interview last night. “It should only be moving forward. … Students should be measured by the standards they were told in the beginning. A new policy will be used moving forward.”

He added: “I apologize to folks who were impacted by this. We should not be in the business of hurting students. We’re in the business of helping students.”

He said the policy was created in the first place to help students who were challenging themselves with the most rigorous courses. It was also created, he said, to make sure grading systems were uniform throughout the city.

“I think there needs to be uniformity,” he said. “And I think the disjointed policy in the past that had been in use needs to be changed. … Going forward we will have one standard, and people should understand what that standard is.”

Mr. Walcott’s stance was a marked departure from what the education department was telling reporters earlier in the week.

On Monday, the deputy chancellor for teaching and learning, Carmen Farina, defended the retroactivity of the new policy. In an interview with the Sun, she said: “Whenever we would make the change – be it now, next year, or two years from now – it would always be retroactive to something, because you can’t have something now without having a past.”

Yesterday’s backpedaling came after about six weeks of pandemonium at city high schools, which erupted when principals started printing out seniors’ transcripts as the teenagers were preparing early college applications and noticed that some students had grade-point averages that exceeded 100% and class ranks were jumbled.

Some schools sent colleges averages without weighting, in apparent violation of the new citywide policy.

Other schools devised their own systems. Stuyvesant High School, for example, sent three GPAs for each student: one unweighted, one with the 10% bonus, and one calculated in a way that benefited children who scored well in AP classes and hurt those who didn’t.

Even principals who were in favor of giving extra credit for the advanced courses opposed the retroactivity of the policy in its original form.

The principal of Brooklyn’s Midwood High School, Steven Zwisohn, told the Sun he found out about the changes this fall when he logged into the grading system and found that all of a sudden his students’ grades were weighted.

Mr. Zwisohn said, he doesn’t object to the policy in theory. Indeed, back when the Department of Education was the Board of Education, he asked his superintendent if he could give a bonus to students who took AP classes. But he said he objected to the way the education department originally implemented the plan.

“We’re for it when we can tell the students and the parents in advance of doing it,” he said. “To put it on after the fact, it surprises students, parents.”

It turned out yesterday that the decision was passed through the department’s technological staff, which was implementing the new high school data system – not through the normal channels. Ordinarily, principals are informed of policy changes in the Principal’s Weekly.