Every consumer knows how the law of supply and demand affects prices. When demand is higher than supply, the price goes up. As suppliers increase production to match the demand, the price goes down.

I've frequently lamented how school districts try to exempt themselves from this law. When districts are reluctant to raise the salary to teachers in fields with more demand than supply, such as ELL, special education, and secondary math and science, it is no surprise when they fail to find qualified candidates with these much-needed skills.

Teacher unions and other opponents of salary differentiation have to exaggerate the breadth of this teacher shortage into claims of a general teacher shortage so they can then justify demanding general policy changes. Of course, raising the salary for all teachers, even those in fields with plenty of applicants, is an expensive way of solving a limited problem.

Still, even when districts ignore supply and demand, others don't. College students, for example. News about teacher hiring and firing does influence undergraduates when they're trying to decide on a major. A few years ago, during the Great Recession, schools dismissed 220,000 teachers--most of them relatively new hires. People with jobs held onto them tightly, reducing the replacement rate. It's no surprise that college students decided against a teaching career, resulting in a drop in enrollment in teacher education of 36 percent between 2009-10 and 2013-14.

Today's college students are hearing a very different story of teacher shortages and plentiful hiring. There's now some new evidence that this is affecting their choices exactly the way the law of supply and demand would predict.

For instance, the California Commission on Teacher Credentialing found that state enrollment in teacher education increased nearly 10 percent between 2013-2014 and 2014-2015. In Indiana, the number of newly licensed teachers increased 18 percent between 2014-2015 and 2015-2016. It turns out that publicity around teacher shortages actually makes them less likely to occur.

The bad news is that we still won't do what it takes to address those persistent shortages of teachers who can fill certain subject areas or work in a rural or high urban school. That's because both higher ed institutions and school districts continue to break the law of supply and demand and should be cause for targeted recruitment strategies, not general alarm.

For instance, higher education programs continue to prepare about twice as many new elementary teachers as are needed to fill openings. They should steer some of these candidates to fields like special education where supply is lower than demand.

There's no need to lower standards at teacher preparation programs to bring in more teachers. Leave it up to the law of supply and demand to rectify that problem. Instead, states, higher education, and districts should solve the actual, more limited problem of shortages in specific fields by directing candidates away from over-enrolled elementary programs and increasing compensation in specialized areas.

As civics teachers tell their students, everyone is supposed to follow the law.