It sounds like a simple solution to a growing problem: Address teacher shortages by adopting policies that encourage veterans to stay in the classroom while attracting retirees back to the job. While many states and districts are doing just that, they are also finding that their policies can have unintended consequences--such as raising questions about fairness, enticing teachers to retire early, and putting new strains on the retirement system. Such arrangements are also helpful to retired educators, many of whom cannot support themselves on their pension checks alone. And the plans are responding to the need for teachers with expertise in shortage-prone areas, such as mathematics, science, and special education. Some policymakers complain that those programs "entice teachers to retire early," so they can receive their pensions and still earn a paycheck, Michelle Exstrom said, senior policy specialist at the Denver-based National Conference of State Legislatures. To discourage such "double dipping," most states require retirees to stay out of work for a month to a full year before returning. State laws, though, vary on whether retirees can be re-employed, how long they can work, and how much they can be paid before they are penalized. A 2004 study conducted by the National Education Association found that all 50 states allow teachers to return to work on at least a part-time basis without losing their pension benefits after they have retired. Thirty-four states let teachers return to work full time. Often, however, districts must meet certain conditions--such as proving that a teacher shortage exists.