Would you choose a mechanic just because he wrote a good essay about brakes?

Would you trust a surgeon just because she could correctly label the kidneys on a diagram? Would you rely on help from a firefighter who'd only read about fires -- as long as an experienced firefighter was looking on?

You'd probably want more than that: strong skills, well-tested habits and meaningful experience.

So why wouldn't you want the same things from a teacher?

The Wisconsin State Journal recently endorsed Senate Bill 175. If passed, it would change the rules for math and science teacher licensing in Wisconsin.

We strongly disagree with the endorsement.

While this legislation is well-intentioned, it will ultimately do more harm than good -- and it is the children in the most troubled schools who will pay the price.

Here's why: SB 175 is intended to attract math and science professionals (engineers and scientists) into teaching, based on the belief that they have the necessary subject-matter knowledge. The bill would allow them to get teaching licenses almost entirely on the basis of written tests (a math test, for example), as long as they receive some loosely specified form of mentoring during their first year on the job.

There's nothing wrong with using written tests, and mentoring new teachers is a great idea. But neither is sufficient to protect children from dangerously under-prepared teachers.

Although subject-matter knowledge is essential to good teaching, the knowledge required for teaching is significantly different from that used by math and science professionals. A well-constructed certification program gives beginning teachers a crucial knowledge base (of math or science as well as about teaching) and helps them develop the skills and practices that bring this knowledge to life.

There's a reason that so many certification programs immerse new teachers in classroom tasks gradually: It gives them a chance to make their mistakes and sharpen their skills in more controlled, lower-stakes contexts before handing them primary responsibility for a classroom of students.

These days, it's popular to argue that education needs some good old-fashioned market principles. Bring in a lot of new teachers, the argument goes, and weed out the ones who fail. The problem with this logic is that actual students are sitting in the classroom of every under-prepared, failing teacher. For these children, the consequences are very real.

Alternative certification bills, such as SB 175, usually place more "alternatively certified" teachers in struggling schools than they do in schools that are doing well. This means putting more under-prepared teachers in classrooms full of students who have fewer academic resources to start with.

Some of these teachers will succeed against the odds. Many will not.

Parents of children in successful schools should ask themselves this: Would I take this risk with my child's teacher? If not, why would I accept that risk for someone else's child?

Hewson is a professor of science education at UW-Madison. Knuth is a professor of mathematics education at UW-Madison.