There are several reasons why your students might experience log in issues:
1) If your classroom internet connection is weak or has a problem.
2) If your students' devices are not running the latest version of Handouts and iOS/Android.
3) If your school has not whitelisted the following domains:
4) If the students registered on iOS they will not be able to access their accounts on an Android tablet, and vice versa. The iOS and Android systems are not compatible at the moment, but we are working on this as we speak and hope to have the functionality ready for early 2015.
5) If you add students manually when using the Google or Edmodo login, students might experience log in issues even if you use the same email used for these services. If you already did this, please write to us at firstname.lastname@example.org giving us the the usernames in question and we will gladly reset the accounts.
If none of these seems to be the problem please get in touch at email@example.com, letting us know the account details for the student(s) in question and what system they registered on (iOS/Android). We will look into the problem as soon as possible.