Is this one field with multiple check boxes or mutliple fields with one check box value for each field? Multiple Records, with a single value check box for recording attendance would actually be best.
For a script to select a single value check box, the script need only set the field to the exact value specified in the value list.
If your value list is a single value that reads "Monday", then:
Set Field [YourTable::attendanceField ; "Monday" ]
will cause the field to display a selected check box.
If you have multiple values for your field (multiple check boxes to one field), you set up code such as:
Set Field [ YourTable::attendanceField ; If ( IsEmpty ( FilterValues ( YourTable::attendenceField ; "Monday" ) ) ; List ( YourTable::AttendanceField ; "Monday" ) ]
This expression appends the specified value in quotes to the return separated list of values already selected in the field. The If function makes sure that if a check box has been selected already, it can add the value to the list twice--which can negatively affect the user's ability to clear a check box previously selected.
Hint: If this is a multiple check box field, try putting a copy of that field formatted as an edit box next to the check box field as an experiment. Then try clicking and clearing different check boxes. The edit box formatted copy will show you what data is actually stored in the field.