The other issue, although I don't know how much it applies in Louisiana, is that they simply may not have enough qualified teachers to fill all of the positions. For certain areas, it can be difficult to find teachers with the right certification and letting them get certified on the job is a compromise. Having all of the teachers certified first would obviously be better, but may be difficult to accomplish.
Indeed, and I think that's where there's a high chance of push-back from the teachers and ISDs. It becomes a chicken-and-egg situation... you can't hire gifted teachers without a cert, but you can't get them to pursue a cert without offering them a job that uses it.
