Screen is left locked
An iPad is locked by some teacher and cannot be used in the coming class.
It is quite possible some teacher has left a class without releasing the lock.
You as a teacher can get into the class and release the lock.
1. Make the iPad off-line so that the lock was released temporarily.
2. Confirm a subject name shown at top right.
3. Get another iPad, login as a teacher, get in the class, and release the lock.
__ If you are not the one who locked the screen,
__ 3'. Get another iPad, login as a teacher, select "Other Teachers' Subjects", select the subject, and release the lock.
4. Make the student's iPad back on-line.