Is your student being asked to pay and upgrade to continue to use Doodle? If so, it is likely they have created duplicate accounts and are not using the school provided account that you created for them.
To resolve this:
Login to the Teacher Dashboard
Head to Manage > Students and select your class from the drop down menu
Find your student's account and here you will be able to see the username that the student should be using
Check with the student that this is the username they are using. If they are not sure what the password is and if you have no record of it you can reset the password by clicking on pencil icon int he edit icon and her you can input a new password for the student