If a student leaves group A and joins group B, he can still join A whenever he wants, since he knows the join code. He can use A's join code to spy on his old group A, or plagiarize their solutions.
If a student leaves group A and joins group B, he can still join A whenever he wants, since he knows the join code. He can use A's join code to spy on his old group A, or plagiarize their solutions.
A quick fix would be to generate new join code whenever a student leaves the group.
Then again, there's one use case for a student flipping between groups described here: https://bitbucket.org/cse-fire/fire/issue/51/student-cannot-resubmit-after-changin-the It would be extremely annoying to constantly ask a groupmate for the new join code.