Here are some troubleshooting steps that you can try:

1. Check your notification settings in Microsoft Classroom, to make sure they are enabled.
2. Make sure you have set the notifications to display on your lock screen.
3. Check your phone’s volume is not set to silent or vibrate only.
4. Open the Microsoft Classroom app, go to the Account page, and select Notifications.
5. Check that your account is connected to the app.
6. Try reinstalling the app to resolve any possible glitches.