Did you know just by having the Telerik.Web.UI in your project you have instant jQuerification! Teleriks client-side functions apparently heavily use jQuery so they rely on it being there and thus embed it into the WebResource. Which means you don't have to make a separate request to include it on your site.
The only tricky part is to use $telerik.$ instead of just $.
If your heart is set on just using $, then try this
var $ = $telerik.$;
So for example to add a class to element with the id "test" use
$telerik.$('#test').addClass('big');
Checkout all the $telerik object goodies here https://www.telerik.com/help/aspnet-ajax/telerik-static-client-library.html