Economic necessity, Mr. Parkes said, has forced some stay-at-home mothers to go back to work. “People are so busy trying to stay afloat, they just do not have as much time as they would like to give,” Mr. Parkes said, adding that he has heard similar laments in regional PTA meetings. “This seems to be a problem for a lot of schools.”

(Mr. Parkes has been getting creative. He reached out to the Cub Scouts to help with the fall festival and recruited the girls’ lacrosse team at the high school to operate the crazy-hair and face-painting stations. He is also exploring arrangements with the R.O.T.C. and with corporations that have public service programs.)

Other forces are at work besides the lack of free time. The growing world of mom blogging has provided ample forums for exposing the darker feelings of motherhood, and a number of women have taken to cyberspace to gripe about school volunteer work. Some complain that the system preys on maternal guilt and that it creates a sense that a mother’s worthiness is measured in how many hours she puts in at her children’s schools. Under the headline “Just Say NO to Volunteering,” Sarah Auerswald, a former PTA president in Los Angeles, wrote in June, “What I am about to say is not very PC, so get ready: Moms, stop volunteering so much.”

Ms. Auerswald, who estimated that she had sat through 1,000 meetings over the last 10 years as a volunteer, said all her work for the schools had left her “a run-down, crabby, resentful wreck.” Worse, she said in an interview, “My kids got really resentful.” When she would leave them with yet another baby sitter, or drag them along for yet another Saturday Clean-up Day at school, they implored, “Why is it always you who has to do everything, Mom?”

Ms. Auerswald emphasized that her children’s school had a very real need for parents’ volunteer work. But she said she has learned that parents need to set realistic expectations about what they can accomplish and how much of themselves they can give.

Because the work is unpaid, some volunteers say, few realize the toll it can take on people. “I know a woman  the work she did for the public schools was so critical  she made me look like a loafer,” Ms. Auerswald said. “Then her husband left her because she was never home.”