The app works for any student.
Premium Students have their own login.
Free users need to use the Find A List (search) function to find their teacher or parents homepage with the lists. It works best if you know your teacher/parent's username.
For teachers or parents who want to do account administration (save lists, import lists, add students etc), it must be done through a browser.
I hope this helps. We're going to try to make this clearer in the app when we updated it.
Of course, like the whole website, the real value is in the premium members. The people who use the free version are really just dabbling around the edges.