• Create a course and enrolled 10 users, 5 into Group1 and 5 into Group2.
• Create a scheduler activity and restrict access it to Group1 (this works)
• Enrol another user with the role of 'Teacher' - This user will not show in the scheduler as a user who is awaiting an appointment
• Add the role of 'Student' to that user and they appeared in the scheduler.
• Remove this user from the 'Teacher' role and left with only the 'Student' role, they don't appear in the scheduler as requiring an appointment.

So it seems that if a user has the role of 'Student' and another role, the scheduler ignores the group restriction.