When students register, as a part of protecting their privacy, they are required to enter a teacher’s invite code. If students register and do not do this, they may be able to create an account but will not be able to tag. To avoid this, make sure all students enter their teacher’s invitation code at sign up.
If a student forgets to enter the invitation code upon registration they can enter it afterward, then click the button to request their teacher manages their password. The teacher can accept the student's request from their student list.