The reason the Teacher is not seeing these Students, to be able to take Attendance is because the Students more than likely are not enrolled into the Course until a future date. 
To see this navigate to:
  • onCampus > Procedure > Academics > Course Sections
  • Click Manage Sections next to the Course Name
Check for each Students Enroll Date, if is for the Future, follow the below steps.

To change this, you can drop the Students from this course while in this Window,
  • Click on Error for each Student
  • Click Save Section
  • To add the Students back in, type in the Students names 1x1 in the Student Type a Name box, click on their name and input a date (use the same date as the other Students unless you need these to be different) 
  • Click Save and Close