Rewarding teachers for their students’ performance seems, at first blush, like an eminently logical course of action.

After all, if one teacher’s students excel, while another teacher’s students struggle with the same material, it just makes sense to give that first teacher more money—both as a reward, and as a wake-up call to the other teacher.

Unfortunately, while this approach may work well on the factory floor, where productivity can be easily quantified, transferring it to the classroom is problematic at best.

Last week, Gov. Bob McDonnell announced that teachers in 13 school divisions across the commonwealth will participate in performance-pay pilot programs during the 2011-2012 and 2012-2013 school years—adding Virginia to a growing list of states taking this approach.

In Virginia’s pilot program, 40 percent of a teacher’s evaluation is based on student academic growth. Teachers in the 25 participating schools are eligible for ...