Parents at the school knew nothing of the episode until one of them, Rachel B. Laiserin, came upon a mention of it while perusing a Department of Education online report earlier this week while reading about PCBs in another school.

On Wednesday, in a letter to parents, the deputy chancellor said parents should have been told.

“We apologize for not notifying you of this sooner and we will work harder to ensure our notices concerning such incidents are sent promptly to our schools communities,” Ms. Grimm wrote.

Ms. Grimm said the lighting fixture had been removed and replaced with a safe unit.

Under pressure from parents and environmental groups, the Department of Education agreed two years ago to spend more than $700 million to replace light fixtures containing PCBs in 800 schools. The city said the process would take up to 10 years, upsetting some who wanted it done faster. Ms. Grimm’s letter said that a buildingwide replacement of light fixtures at P.S. 87 was now being accelerated.

But parents, elected officials and the teachers union say they are not satisfied and plan to hold a news conference Friday outside the school, on West 78th Street, to demand more answers. In a statement, Assemblywoman Rosenthal said city law requires that parents be notified of such episodes within seven days.

Marge Feinberg, an Education Department spokeswoman, said, “We understand our school communities are very concerned by such incidents, and we continue to actively work with the Environmental Protection Agency to improve our existing protocols around PCBs, including notification to parents.”