I have a suspicion about the cause of the problem. The ASP.NET validators are not AJAX controls and they don't register their scripts through the ScriptManager. As you're showing the scheduler with an AJAX request, their scripts are not getting loaded.
My suggestion is to place a validator on the page and hide it in a div with display:none. Any type should do, just set its ValidationGroup to something that is not used on the page, so it'll not actually validating anything.
I hope this helps.
the Telerik team