Authorities in California are said to have been alerted by students at the school, but the cookies have yet to be tested.
The Da Vinci High School declined to comment on the story, but said those involved were “remorseful”.
The supposedly ash-laden sugar cookies were handed out to nine students, police said, but they were unsure of how many students were involved in distributing them.
One student said that the girl told classmates that the cookies she had given them two weeks ago had contained her grandmother’s ashes.
Da Vinci High School principal Tyler Millsap said there was “no health risk to our campus or to any one of our students” in a letter sent to parents.