If you're using Google Classroom in Classwize and notice that the students you added disappeared the next time you opened the class, this is due to syncing.
Syncing Classwize with Google Classroom will override any changes you made in Classwize using the Modify Students feature in Class Tools.
To permanently change student enrollment in your class, update them directly in Google Classroom. If you don’t have access to add or remove students from your Google Classroom, contact your school’s IT Support for assistance.