Arizona Governor Doug Ducey has pledged to give teachers a 20% raise by 2020. But teachers say that won't cut it.

They voted for a statewide walkout, which will start next Thursday if their demands aren't met before then.

They are asking for pay raises and increases to school funding every year until both hit the national average.

Their demands specify a 20% raise immediately next year. But that wouldn't even bring their pay up to the national average. At $47,403, the average Arizona teacher's salary has fallen more than 10% since 1999 when adjusted for inflation. The national average is currently $58,950.

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Meanwhile, Arizona teachers have had to kick in a larger portion of their pay to the pension fund. They contributed about 2.2% back in 1999 and now contribute 11.3% of their pay, according to data from the Arizona State Retirement System.

"This is unusual," said Keith Brainard, the research director at the National Association of State Retirement Administrators. "Most public employees contribute a fixed percentage of their pay," he said.

Instead, Arizona teachers have to contribute more when investment returns have lagged, sharing more of the investment risk than workers in other states generally do. But their contributions have increased despite a rising stock market. Worker contributions also go up in Arizona when the plan lowers its expected rate of return, which it adjusted this year.

The state contributes about the same amount to the pension fund as the teachers, but that's also unusual. Employers, on average, contribute a bigger share, Brainard said.

Arizona teachers are also covered by Social Security (teachers in 15 other states are not). That takes another 6.2% in payroll taxes out of their paychecks.

After deducting for the pension contribution and Social Security, the average Arizona teacher's pay is reduced to about $39,800 a year, before taxes or health care benefits.

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Arizona Education Association President Joe Thomas said the teachers are calling on the legislature and governor to meet their demands before Thursday's walkout.

"Give our students the schools they deserve. Give us the schools we want to work in. And give us the schools that will stop educators from leaving our state and teaching somewhere else," he said when announcing that 78% of teachers voted in favor of the walkout.

Some Arizona teachers were protesting for higher pay and better school conditions earlier this month. Last week, Governor Ducey announced a plan to raise teachers' pay gradually so that they'll see a 20% raise by 2020, but support staff were left out of the raises. The plan also called for restoring funding to schools.

"No one wants to see teachers strike. If schools shut down, our kids are the ones who lose out," Governor Ducey said on twitter after teachers announced the walkout.

"We have worked side by side with the education community to develop a sustainable plan to give teachers a 20 percent raise by 2020. I am committed to getting teachers this raise and am working to get this passed at the legislature," he posted.