Greetings, acooke33!
Thanks for reaching out to the Apple Support Communities! It sounds like you have an issue with one student being able t unlock their iPad after it's been locked through Apple Classroom. I'd suggest reviewing the info in this article to View and lock a student’s screen with Classroom and verify settings for the MDM configuration:
"Note: Depending on how your mobile device management (MDM) solution is configured, you may not be able to perform this task. Contact your MDM administrator and have them review Classroom view and AirPlay management."
If you continue to have trouble, or need further assistance with configuration, I would recommend reaching out to Apple directly by checking out this link to Contact Support and they will be more than happy to proceed with additional steps.
Take care!