Heather Gillette, president of the Parent Teacher Student Association (PTSA) of Cascade Ridge Elementary School, isn't sure exactly when members started opening their checkbooks. But Gillette says that over the past decade the parents of children attending this small primary school in Issaquah, Wash., have helped to pay for enrichment programs in subjects such as art and science, and for educational assistants or teacher aides, who help regular staff on an ongoing basis.

On nearby Mercer Island, a similar parents' group initiative also has resulted in enough money to fund teacher aides in some elementary classrooms. And, parents in neighboring Bellevue have ponied up money for tutors, science teachers, and reading specialists across the district.

"The PTSA here has not been about bake sales or bringing a fun carnival to the school in a very, very long time," says Gillette. She calls the Cascade Ridge PTSA "a watchdog over basic education," whose goal—backed up by direct financial donations—is "making sure children are exposed to different experiences and instruction."

These wealthy Seattle suburbs are not alone. As public school districts across the country face tighter budgets and higher expectations, worried parents are increasingly trying to fill the gaps between where schools are and where they need to be. Private funding for public K-12 schools is the most rapidly growing philanthropic sector, according to Howard Schaeffer of the Public Education Network (PEN), a national association of education funds in low-income communities. How fast this trend is growing is hard to gauge, Schaefer says, in part because there are thousands of groups that raise less than the $25,000 threshold for IRS reporting, but its impact is unmistakable. "People are starting to see that K-12 education is a worthy, effective place to put their charitable dollars," Schaeffer says.