You should set the src attribute after the onload event, f.ex:
el.onload = function() { //...
el.src = script;
You should also append the script to the DOM before attaching the onload event:
$body.append(el);
el.onload = function() { //...
el.src = script;
Remember that you need to check readystate for IE support. If you are using jQuery, you can also try the getScript() method: http://api.jquery.com/jQuery.getScript/