This is more of a long-term strategy that yields results over time, but I've found that typing helps a lot. Think about passwords you have to type all the time: even if they're nonsense combinations of letters, numbers, and symbols, your muscle memory takes over and you can spell them accurately. So too for real words! I'll frequently find myself writing out a word, not remembering how it's spelled, and then closing my eyes and typing at an imaginary keyboard, letting my fingers find the right letters without my higher brain getting in the way and second-guessing.
So, if your child/student is pretty comfortable and familiar with using a keyboard, you might consider just having them type the correct spellings of the words over and over and over again to develop that muscle memory. It's not for everyone, but it may be worth considering.