Limit your found set or related records to only those with "pass" in the pass field.
If this is a summary report, perform a find for just those records. If you are referring to this field from a related table, include the Pass field in the relationship so that only students that have passed are included in the relationship. This typically requires adding a calcualtion field with a constant value of "pass" to use in this relationship.
Hmm, had another thought. as what I suggested messes up the second part of the calculations (the count of total students).
Is this a value you are putting in a sub summary part sorted by teacher?
If ( Pass = "Pass" ; Round ( GetSummary ( count ; Pass )/GetSummary ( count ; Teacher ) ; 3 )*100 ; 0 )
The IF statement did it!
(the other option - performing a find for a subset of the records - would not have been useful, because I wanted to report on all of the teachers' pass/fail and achievement level breakdowns at once)