BOISE — Idaho’s teachers union Thursday questioned the state schools chief’s portrayal of student-teacher ratios as debate about overhauling public education continued.

Superintendent of Public Instruction Tom Luna wants to increase the state’s average number of students per teacher from 18.2 to 19.8 and put the money saved into the core initiatives of his technology-heavy “Students Come First” plan.

Idaho’s student-teacher ratio would increase through a gradual elimination of about 770 teaching jobs through attrition and a two-year uptick of nearly two students per teacher in grades four through 12. The student-teacher ratio in kindergarten through the third grade would remain unchanged.

The question now is what would be gained and lost by increasing that mark.

Like many statistics, there are multiple ways to look at the proposed change.

First, there’s a difference between student-teacher ratios and average classroom sizes. Many teachers lead more than one classroom. That’s something the Idaho Education Association, the state teachers’ union, is quick to point out.

“The reality is that there are many teachers out there who are working with a whole lot more students than that (18.2-student mark), so it sounds very simplistic to say, ‘Just add two more kids,’” said IEA President Sherri Wood.

She added: “It’s about how many kids can we educate and giving them the needed attention in order for them to be successful in the classroom. It just baffles me when I hear policy makers say there’s no research. Obviously they haven’t spent time in a classroom with 35 students when you’re attempting to teach them something.”

In a Thursday phone interview with the Times-News, Luna said there’s no evidence to suggest that larger class sizes hurt student performance.

“There is no credible study that decreasing class size has a positive effect on student achievement or increasing class size has a negative effect,” Luna said, adding that the most important factor is the quality of teachers and principals.

Luna said he’s “been surprised at the number of very positive e-mails from teachers” he’s received about his proposal and drew a distinction between unions and the educators they represent.

“There’s a huge difference between union leaders and classroom teachers,” he said. “I think it’s in their (union leaders’) DNA to oppose anything we propose.”

Idaho already has the seventh-highest student-teacher ratio in the nation, according to a National Center for Education Statistics report based on 2008-09 data. The six states above it are Arizona, California, Nevada, Utah, Oregon and Washington.

Twin Falls School District Superintendent Wiley Dobbs said he likes that the ratio for the lower grades would be untouched because that period is crucial for learning basic skills.

“I love the idea of helping all the other teachers get back on the salary schedule they’re supposed to be on,” said Greg Lowe, superintendent of Wendell School District. “But how does that affect our district as a whole with having less teachers and higher class sizes? … I’d have to look at that and see how that would affect us.”

Ben Botkin may be reached at bbotkin@magicvalley.com or 735-3238.