privilege set problem

I've got a rather complicated situation that I need help with - let me see if I can explain.

I have a database called student_information designed to house confidential information on students. Only teachers of a given grade level should be able to access student information records in that grade level.

The information records are not kept in a table in this database. The records are kept in a separate admindata file. This data file is the main student management file for the school which has several tables including:

students - all student demographic info

contacts - contacts associated with students

teacher - teacher demographics

class - class information

class data - which students are assigned to which classes

student info - my data records

I'm trying to create a privilege set in the admin datafile that limits records viewed based on the grade level. I've limited the viewing of records based on the calculation: grade_level = "4th". The grade_level field is a calculation field- Last(Class::Grade).

As long as I try to use the grade_level field based on the Last(Class::Grade), I can't view any records. When I changed the field to a text field and entered the same data, the records were correctly limited.

I've tried giving view only access to the class table and all of it's fields - no improvement.

Is it possible that you can't write a calculation to limit records viewed based on a calculation field? What else am I missing here?