The measure, which passed near the end of Tuesday's marathon House session, would allow up to 32 students in high school math, science, social studies and language arts classes.
It's a step back from legislation that passed seven years ago to reduce high school class sizes to 28 students.
State Rep. Brooks Coleman, a former principal who sponsored the bill, said he wants to give school boards more flexibility to gradually reduce the class sizes. But education groups called the move shortsighted, and vowed to fight it in the Senate.
"It's not only an academic issue, it's a school safety issue," said Tim Callahan, spokesman for the Professional Association of Georgia Educators, a group of 69,000 teachers.
"We'd have more personal instruction, more orderly classes. But unfortunately, the Legislature has chosen to delay."
Jeff Hubbard, the president of the Georgia Association of Educators, said his group is "very disturbed" by the plan.
"We keep talking about improving Georgia education," he said, "and one of the clear ways is to have lower class sizes."
Supporters say the proposal helps school districts deal with growing attendance rolls, while critics argue that the state could use its budget surplus of more than $500 million to hire more teachers and reduce class sizes.