The attorney, Terrence Connors, said the defendant, Peyton Gendron, would enter a guilty plea on Monday to all 25 counts against him in a state indictment at a 2 p.m. hearing in Erie County Court in Buffalo.
He initially pleaded not guilty to all counts in June.
The guilty plea would mean that the case against Gendron, who is from Conklin, New York, will not go to trial.
He could face life in prison or the death penalty on the state charges.
Gendron also faces 27 federal hate crime and firearms charges, to which he pleaded not guilty to in January.