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 the class, you can change the class code to prevent additional students from joining with the old code.
To start, go in to the class you need. Tap the wrench icon and open the class settings. Look for the circle-arrow button to change the class code.
When you need new students to join, communicate the new code to them – never on Social media or a public-facing school website. Students that are already joined the class will remain.