int PCount;

AnsiString szSQL = "SELECT STUDENT_ID, STUDENT_NAME "
                            "FROM TB_CLASS "
                            "WHERE CLASS_ID IS :parmClassID "
                            "ORDER BY STUDENT_ID ";

if(qryParam->Active) qryParam->Close();
qryParam->SQL->Clear();
qryParam->SQL->Add(szSQL);
qryParam->Parameters->AddParameter();
PCount = qryParam->Parameters->Count - 1;

qryParam->Parameters->Items[PCount]->Name = "parmClassID"; 
qryParam->Parameters->Items[PCount]->DataType = ftString;
qryParam->Parameters->Items[PCount]->Direction = pdInput;
qryParam->Parameters->Items[PCount]->Value = "A-1";
qryParam->ParamCheck = true;
qryParam->Open();