If you are unable to connect to another player's world and they can't connect to yours, try the following fix:

Open keychain access, then toggle "hidden items" from the view tab.

1. Quit Minecraft Edu
2. Search for entries with the name "PF_Account_6955F" and "PF_Account_6955F_Key". Delete these.
3. Finally, search for one more entry named "AD_EDU_USER_HINT" and delete it. Retry joining a world. You may be prompted to login again, though.