Small method for saving student attendance
First of all, minor beef. I'd reccommend that you change your index names to be more descriptive, particularly the inner one. For instance you could make your repo query

DateTime date = new DateTime(year, month, **day**);


Much more readable. When an index has a name, name it.

If you're having performance issues, try separating out the queries. In psuedocode:

foreach day in month
   get students_with_records from database

   foreach student in the_class
      if student NOT in students_with_records
         add record


This method will put the search through the student records on the client...it should be faster, since you will only have one query per day rather than one query per day per student. You really should profile the app and see if it's an issue, though.