Signing in for students

I am running our STEM program K-5 and I have teachers that are having issues with students logging in from beginning of the year. Students are not able to log back in after registering for their course section. When they do log in, their material is not saved.
What is the best way to stop this from occurring? It is multiple teachers at multiple campuses. We are a google school. Should the students log in with google and then put in their course code?