Please be mindful of who you're sharing the class codes with. Keep a close eye on the class members so you can monitor the students that are joining.
Once all your students are joined to the class, you can remove the class code to prevent any additional students from joining. This will lock the class and adds a level of security to your classes to ensure that only the correct students are joined.
To start, go in to the class you need. Tap the wrench icon and open the class settings:
Look for the "X" button to remove the class code. Once removed, you'll see a message that no other students can join until you refresh the code:
When you need new students to join, tap the blue restore button to restore the class code. A new class code will be generated meaning that the previous code will be invalid. Students that are already joined in the class will remain and restoring the class code will allow new students to join.