You could have separate lists for them on your account, although I know that would be a lot of work to personalize each list, because you wd not want to give out your password to the students. Just name the lists by the kids' names.
Another suggestion so that the students would have access to edit and change the accounts would be to create email accounts. Yahoo and gmail are free, and they would not have to USE these emails, or even know what they are. They would just need to sign into SpellingCity under their username and password.
Hoping you'll find a solution that works well for you!
Homeschool Mom,
martyh