You may encounter the situation where the class shows up on the timetable but not on the attendance. In this example the class is AVI10a and it takes place every day on period 3: 



The attendance doesn’t even give the option to mark the student absent because it doesn’t see any class there. To fix this, from the main menu go to Setup- Class Info- Classes- This Year, search for the class and click edit. We can see the issue right away here, being that the class start date is a future date: 


This is why the attendance doesn’t see the class since according to the system it hasn’t started yet. Fix this date to the proper start date and then go back into the student record and you will see the class available to edit in attendance: