Screen Lock

I think when pupils move away from the bluetooth range of the teacher device (even when on the same wifi) the controls of classroom are disabled. This might also be similar if the teacher device goes to sleep in the lesson, I usually keep my teacher iPad plugged into power, with Classroom open and screen settings set to always on.

I've seen this happen if students use the iPad buttons to do a hard reset/restart of the iPad after you send the lock command. When the iPad restarts, it doesn't stay locked. It's open because it disconnected from Classroom. If you notice this happening, you can watch for the iPad to reappear in Classroom and then send the lock command again for that student. I've had some smart elementary students figure this out and do it intentionally while teachers are busy with instruction or assisting other students and not watching the Classroom screen.

Being new to Classroom, I'll also share that when you see students as "offline," check to see if they've turned off Bluetooth, turned off WiFi, or enabled airplane mode. These actions will make the iPad appear "offline," and you can't see it or send commands. Figuring out why students show as "offline" can be one of the most frustrating aspects of using Classroom.

If your technology department manages the iPads with an MDM (mobile device manager), they might be able to lock the settings so that students can't make these changes.

Apple Classroom is amazing, so it makes me happy that you're using it! My favorite feature is being able to force all students to a web site at once. They no longer have to try to navigate there themselves. It's a gamechanger for elementary classrooms where students haven't developed great typing skills yet.