var Marker = "true";

 function CheckboxenAktivieren() {

  if(Marker == "false") {

   for(i = 0; i < (document.getElementById("button_form").length)-2; i++) {

    document.getElementById(i).checked = true;

   }

   Marker = "true";
   return "Auswahl aufheben";

  }
  else {

   for(i = 0; i < (document.getElementById("button_form").length)-2; i++) {

    document.getElementById(i).checked = false;

   }

   Marker = "false";
   return "Alle makieren";

  }

 }