More than 30 children and their parents carried the signature pages into the secretary of state's office in Olympia for review. About 246,000 valid signatures are needed to qualify for the November ballot.
"Over 325,000 voters are delivering a clear message that large class sizes are unacceptable, and we must give our kids the attention they need to succeed," said Class Size Counts campaign manager Mary Howes.
Class sizes vary across the state, but sponsors say Washington has some of the largest average class sizes in the nation.
If voters were to approve Initiative 1351, it "would make us about average," Howes said.
For kindergarten through third grade, non-poverty schools average about 25 children per class. The initiative would drop that number to 17.
Grades four through six average 27 students in both poverty and non-poverty schools. The initiative would reduce that to 25 in non-poverty schools and about 23 in poorer schools.
High school classes would go from nearly 29 students on average to about 24.
Lowering class sizes would cost the state billions of dollars in teacher salaries and school construction. The initiative does not include a plan for finding that money.
The Legislature already has committed to similar class size decreases. Those proposed changes were reinforced as part of the state Supreme Court's McCleary school funding ruling and would cap kindergarten through third-grade classes at 17 children. Other grades would be limited to 25 students.
Initiative sponsors say one of the reasons for their effort is to force the Legislature to make good on its promises to reduce class sizes.
"The bottom line is it helps all students succeed," Howes said of the initiative. "All students deserve to succeed in an uncrowded classroom."
Howes said her group gathered all of the signatures through volunteers. She said the campaign paid some signature gatherers just in case they needed help, but didn't turn in any of the signatures they collected.
"I'm just so proud of all of our volunteers and all the work they've done. It's really an accomplishment," Howes said.