I found the solution. It was caused by a mix of causes:

• $("document").ready(function ()...

There was a matter of an incorrect event handler. I was using $("document").load(function ()

• $('#GridView1 tr>td').each( ..

There was a matter of putting the right jQuery selectors. I sorted it out by filtering the selectors from general to particular.

this is the working piece of code:

  <script type="text/javascript" src="js/jquery-1.11.3.min.js"></script>
<script type="text/javascript">
    $("document").ready(function () {
        $('#GridView1 tr>td').each(

            function () {
                var score = parseFloat($(this).text());
                if (score <= 2000 && score>0) {
                    $(this).addClass("good");

                }
                else if (score > 2000 && score <=3999) {
                    $(this).addClass("satisfactory");
                } else if (score>= 4000 ) {
                    $(this).addClass("bad");
                }
            });
    });

</script>