The complaint comes after the Public Interest Law Center of Philadelphia completed an analysis last summer of the issues at Parents United's request.
Center attorney Michael Churchill said the failure to register with the city constitutes a violation, not the act of lobbying itself.
"This opinion does not say it was illegal for them to engage in these actions. It says, if they do it, they should register," Churchill said.
William Penn officials said they did nothing wrong.
"We are aware of their intent to file a complaint, and our attorneys are confident that it is without merit. The William Penn Foundation has been a force for public integrity and civic good in the greater Philadelphia region for nearly 70 years, and we will continue that tradition through our efforts to improve children's futures, protect the environment, and nurture creativity and rich cultural expression," spokesman Brent Thompson said in a statement.
Once the city Ethics Board receives the complaint, it will be reviewed, but the timetable is unclear.
In a surprise move, William Penn chief executive Jeremy Nowak left the foundation last week. Nowak was highly involved in the BCG and district work, but board chair David Haas has said his departure had nothing to do with the foundation's school district work.
A BCG official also expressed confidence that the complaint would be dismissed.
"BCG was hired as a consultant to the School District. We were engaged at all times to perform services for the district and for no other purpose. None of our activities on behalf of the district constituted lobbying under the law," spokesman David Fondiller said in a statement.
The School District declined to comment, saying it had not seen a copy of the complaint, but its officials have said repeatedly that they regard BCG's work as a set of suggestions they can adopt or disregard as they see fit.
In an interview, Helen Gym, a founder of Parents United for Public Education, said the group sought the legal opinion because it believed an unprecedented amount of money and influence was being exerted in the School District by people and agencies that have agendas.
"We're talking about millions and millions of dollars. A mass expansion of charter schools. A mass closure of public schools," Gym said.
Everyone appreciates genuine public-private partnerships to help schools, but the district struggles to tell the difference between benign donors and advocates who seek particular outcomes, she said.
"The district itself needs to be aware of what lobbying is or isn't," she said. "It's a legitimate issue for the city Ethics Board to take a look at."
Contact Kristen Graham at 215-854-5146, firstname.lastname@example.org or on Twitter @newskag. Read her blog, "Philly School Files," at www.philly.com/schoolfiles.