Corran School's trust board acted unlawfully when it decided to merge with St Kentigern without consulting parents, says a QC hired by the parents.
Deborah Hollings' report said the board should have contacted the Attorney-General, Chris Finlayson, who would have consulted the community, when it found it didn't have enough funds to carry out the trust's purpose.
Mr Finlayson has the power under the Charitable Trusts Act to inquire into the management and administration of any charity.
The act also enables any person to sue to enforce the charitable purposes of a trust.
The parents plan to approach Mr Finlayson today.
Rod Fogarty, one of the parents spear-heading the legal investigation, said he did not believe it would be in the interests of either board to put the matter of their children's education through a commission of inquiry but has asked them both to allow a 60-day consultation period before going ahead with the merger.
Otherwise the assets and liabilities of Corran will be transferred to St Kentigern on Thursday.
Mr Fogarty represents the views of more than 100 parents, past students, prospective parents and interested community members who are angry about the lack of consultation and hope to stop the merger.
Other parents support the decision to merge and believe it is in the best interests of their children.
Since the board's announcement on April 6, it is understood that Diocesan School has received more than 50 applications from Corran girls who are looking to transfer there rather than to St Kentigern College next year.
And already, instead of returning to Corran after the holiday break tomorrow, a number of girls will be putting on Diocesan uniforms.
Parents have been advised that there are grounds to apply to the court for an injunction, and many, who are experienced litigants, have indicated they would be prepared to do this.
But Mr Fogarty thought this option would put the applicants and the board members at risk of having to cover any losses incurred and could see both boards diverting money from education to a courtroom battle.
He said that the only viable way the boards could avoid a commission of inquiry was to volunteer a 60-day stand-down.