Why not pre-test students so they are only practicing words they don't know?
My DD got mostly words she didn't know this past year. In fact, she mostly got words she had never come across, and was unfamiliar with the meaning of, or had never heard spoken aloud. Unfortunately, there were so many spelling lists among all the kids that there was no in-class instruction. And the spelling tests were administered by another kid, who sometimes mangled the pronunciation so badly that DD thought maybe she'd been pronouncing it wrong, and changed her spelling to match the other pronunciation.
