You do not need a separate table in order to do this.
A summary report using a summary field that computes the total of your absent field is all that you need for that purpose.
You can use sort records to group your records by student ID, month and year with a sub summary part used with the summary field to show a subtotal for each student for the specified month and year.
The individual attendance records need not be visible in this report.
See this tutorial on summary reports: Creating Filemaker Pro summary reports--Tutorial
Phil will that allow you to work with the "new" summaries as if they were calculated fields say to find the number of attended days or percent of days in attendance or absent?
It's hard to answer definitively from such limited information, but yes, calculations using the GetSummary function can compute values that use the subtotal type values a summary field would show in such a sub summary layout part.
But there is also another option that may work for some types of reports.
If you have this relationship:
Students::_fkStudentID = Attendance::_fkStudentID
then a calculation field defined in Students can either use an aggregate function such as count, sum, average, etc or a summary field defined in Attendance to compute values based on the related records linked to that student record. (And there are ways to make that relationship more specific so as to separate out data for the student for different semesters and/or different classes.
ExecuteSQL also can be used to compute such values.