If you look at the documentation for CController, you will find that CController::renderPartial is not a static method, so that error makes sense.
Now for you modified code, there’s nothing wrong with it but normally, it would search for a view file under the views folder for the controller id passed to CController, which in this case your controller view folder might not have a file called _agendaTable.
If your widget extends from CWidget, I think it has its own render function which would search for view files under "widget/views/NameOfViewFile.php".
<div id="schedule-div" class="holder">
<!-- Table -->
// Schedule Table
echo "<div id='user-schedule' class='agenda-table'>";
echo "</div>"; // END "user-schedule"
else echo "<p>You have not registered for any activities</p>";