To load html content of a template inside a controller, for example for ajax response, you can do :

$block = $this->getLayout()->createBlock('adin_admin/adminhtml_scheduletcrender')->setTemplate('admin/scheduletcrender.phtml');
$block->assign(array('ateliers' => $ateliers));
$html = $block->toHtml();
$response = array('html' => $html);
     
//send response as json
$this->getResponse()->setHeader('Content-type', 'application/json');
$this->getResponse()->setBody(Mage::helper('core')->jsonEncode($response));