Percentile in Access

Hi I did a search on the web and found a code for percentile but somehow I'm not able to get it to work correctly bec of the way i've designed my db. Since my select statement involves 2 tables, i not able to use the function. Below is the table structure.

table 1:Student
StudentID Name Class

table 2: Mathematics
ExamID StudentID Scores

So basically I need to get the percentile for a given a class.

Thanks

Public Function PercentileRst(RstName As String, fldName As String, PercentileValue As Double) As Double
   'This function will calculate the percentile of a recordset.
   'The field must be a number value and the percentile has to
   'be between 0 and 1.
   If PercentileValue < 0 Or PercentileValue > 1 Then
      MsgBox "Percentile must be between 0 and 1", vbOKOnly
   End If
   Dim PercentileTemp As Double
   PercentileTemp = 0
   Dim dbs As Database
   Set dbs = CurrentDb
   Dim xVal As Double
   Dim iRec As Long
   Dim i As Long
   Dim RstOrig As Recordset
   Set RstOrig = CurrentDb.OpenRecordset(RstName, dbOpenDynaset)
   RstOrig.Sort = fldName
   Dim RstSorted As Recordset
   Set RstSorted = RstOrig.OpenRecordset()
   RstSorted.MoveLast
   RstSorted.MoveFirst
   xVal = ((RstSorted.RecordCount - 1) * PercentileValue) + 1
   'x now contains the record number we are looking for.
   'Note x may not be     whole number
   iRec = Int(xVal)
   xVal = xVal - iRec
   'i now contains first record to look at and
   'x contains diff to next record
   RstSorted.Move iRec - 1
   PercentileTemp = RstSorted(fldName)
   If xVal > 0 Then
      RstSorted.MoveNext
      PercentileTemp = ((RstSorted(fldName) - PercentileTemp) * xVal) + PercentileTemp
   End If
   RstSorted.Close
   RstOrig.Close
   Set RstSorted = Nothing
   Set RstOrig = Nothing
   Set dbs = Nothing
   PercentileRst = PercentileTemp
End Function

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