Just last week, Barge told lawmakers to expect at least a 10 percentage-point drop as the state begins calculating its graduation rate using a formula that better accounts for dropouts.
But in an appearance before the Georgia Association of Educational Leaders, Barge said a more detailed analysis shows the state's 80.2 percent graduation rate could fall to 64 percent this year.
The graduation rate could take another big drop if students struggling with the state's integrated math curriculum aren't able to graduate with their class, though that could change with some recently proposed changes to the curriculum, Barge said.
Officials have long expected a drop in the state's graduation rate once Georgia switches from a formula that has been in place since 2003 to the federal formula.
Beginning next year, all states will move to the federal method. By 2012, that graduation rate will be a factor in whether a school makes "adequate yearly progress" under the federal No Child Left Behind Act.
Also appearing before the crowd was Gov. Nathan Deal, who weighed in on the debate over future funding for the HOPE Scholarship program.
HOPE is funded through the Georgia Lottery, but it isn't generating enough money to keep up with demand as enrollment and tuition rise.
The scholarship had a $1 billion reserve fund, but that will drop to about $321 million by the end of fiscal year 2012, according to the Georgia Student Finance Commission, which oversees the program.
Deal said he doesn't believe HOPE should be paying for remedial courses for college students.