Problem with Dynamic Selection Screen

Hi ppl,

I have this part of code in my program. But it is not working as desired. Please could somebody let me know what is wrong with it.

TYPES: BEGIN OF tp_selscr,
         klart TYPE klah-klart,
         class TYPE klah-class,
       END OF tp_selscr.

DATA: wa_selscr TYPE tp_selscr.

SELECTION-SCREEN SKIP 1.
SELECTION-SCREEN BEGIN OF BLOCK cls WITH FRAME TITLE text-s01.
PARAMETERS:     p_cls AS CHECKBOX USER-COMMAND chk.
SELECT-OPTIONS: s_klart FOR wa_selscr-klart MODIF ID cls,    "Class type
      	         s_class FOR wa_selscr-class MODIF ID cls
                        MATCHCODE OBJECT clas.
SELECTION-SCREEN END OF BLOCK cls.


AT SELECTION-SCREEN.
*AT SELECTION-SCREEN ON p_cls.

  PERFORM dynamic_sel.  "Selection based on class


*&---------------------------------------------------------------------*
*&      Form  dynamic_sel
*&---------------------------------------------------------------------*
*       Selection based on class option
*----------------------------------------------------------------------*
FORM dynamic_sel.

  IF p_cls EQ 'X'.
    LOOP AT SCREEN.
      IF screen-group1 = 'CLS'.
        screen-active = 1.
        MODIFY SCREEN.
      ENDIF.
    ENDLOOP.
  ELSE.
    LOOP AT SCREEN.
      IF screen-group1 = 'CLS'.
        screen-active = 0.
        MODIFY SCREEN.
      ENDIF.
    ENDLOOP.
  ENDIF.

ENDFORM.                    " dynamic_sel

Regards.