The legal age at which students in Missouri and Illinois can drop out of high school has inched up to 17 in recent years. Now, President Barack Obama wants states to do more.

In his State of the Union address on Tuesday night, he called on every state to require students to stay in high school until they either graduate or turn 18.

But some educators and researchers question the cost and effectiveness of such a measure. And they say that truly addressing the dropout problem requires far more than changing a number.

"We're adamant that we want every student to graduate," said Chris Guinther, president of the Missouri National Education Association. "But what we realize is that every student cannot learn in a traditional high school setting."

School districts need the resources to create environments where every student can be successful and graduate, she said.

In 2009, Missouri aimed to improve graduation rates by changing its attendance requirement from 16 to 17 years old, or once students successfully complete 16 credits toward high school graduation. Illinois made a similar switch in 2005.

But going to age 18 has been met with some resistance. One such bill sponsored by Rep. Jamilah Nasheed, D-St. Louis, failed to move forward last year. Some cite cost as a factor.

More students mean school districts need more money to pay for more teachers, counselors, and classroom space, said Sen. David Pearce, chairman of the Senate education committee. And trying to keep older students enrolled could be expensive to enforce.

"If it's just an idea that's funneled off to the states to take care of and pay for, then I don't think it's a good idea," said Pearce, R-Warrensburg.

Missouri officials could not provide estimates on how much it has cost the state to raise compulsory attendance from age 16 to 17.

Nor is it clear whether the age change has helped curtail dropout rates, which have remained relatively flat in recent years.

Studies have shown that students who drop out of high school will earn less in their lifetime and are more likely to be unemployed. The Alliance for Excellent Education found that dropouts are 3.5 times more likely than high school graduates to be arrested.

"We have to be proactive. We're going to pay on the front end or on the back end," Nasheed said. "We put that money towards having them become productive citizens, or we'll pay to imprison them."

Missouri and Illinois are among 11 states where the age requirement is 17. According to 2010 data from the Education Commission of the States, 20 already states require students to stay in high school until age 18.

Parents or legal guardians are responsible for a student's school attendance, and penalties can include fines and jail sentences.

In Massachusetts, the ongoing debate about raising the age to 17 prompted a study by the Rennie Center for Education Research and Policy that did not find any evidence that simply raising the age requirement would result in decreasing dropout rates and increasing graduation rates.

The study found data in past years showing that of the 10 states with the highest graduation rates, only three states required attendance to age 18. The study also questioned whether the higher-age attendance laws can be effectively enforced. Some state like Illinois include tough measures that tie driving privilege to high school attendance. But the study said it's unclear whether those measures work.

Chad d'Entremont, executive director of the Rennie Center, said addressing the problem is not as simple as identifying the age when students should graduate. Several factors can go into whether a student drops out — their home life, relationships at school and other demands.

Rather, states and educators need to identify students who are struggling, he said.

"If we're waiting to act until the end of a student's career, it's too late," d'Entremont said.