Удаление записей не работает

CustomerData.IBTable1.Active:=False;
CustomerData.IBTable2.Active:=False;
CustomerData.IBTable3.Active:=False;
CustomerData.IBQuery1.Active:=False;

if CustomerData.IBQuery1.Transaction.InTransaction=False then
CustomerData.IBQuery1.Transaction.StartTransaction;
for i:=0 to id_studs.Count-1 do begin
CustomerData.IBUpdateSQL1.DeleteSQL.Clear;
CustomerData.IBUpdateSQL1.DeleteSQL.Add('DELETE FROM MARKS WHERE ID_STUD='''+id_studs+''';');
CustomerData.IBUpdateSQL1.ExecSQL(ukDelete);
end;

CustomerData.IBUpdateSQL1.DeleteSQL.Clear;
CustomerData.IBUpdateSQL1.DeleteSQL.Add('DELETE FROM STUDENT WHERE ID_GROUP='''+id_group+''';');
CustomerData.IBUpdateSQL1.ExecSQL(ukDelete);

CustomerData.IBUpdateSQL1.DeleteSQL.Clear;
CustomerData.IBUpdateSQL1.DeleteSQL.Add('DELETE FROM GROUPS WHERE ID_GROUP='''+id_group+''';');
CustomerData.IBUpdateSQL1.ExecSQL(ukDelete);

CustomerData.IBQuery1.Transaction.Commit;

CustomerData.IBQuery1.Active:=True;
CustomerData.IBTable1.Active:=True;
CustomerData.IBTable2.Active:=True;
CustomerData.IBTable3.Active:=True;