The code is working, but I think the problem is in the logic...
First... do you have in your CSS the class hidden?
.hidden {display:none;}
Second... the submit button when clicked will open a new page so what happens is that you click the button, the button is hidden but then the '/tirociniStage/index.php?r=docente/create' is called and displayed as a new page, so you don't see any more the page with the hidden button..
maybe to use ajaxSubmitButton ?
<div id="clickme1" class="row buttons" >
<?php echo CHtml::ajaxSubmitButton('Save','','',array(
"id"=>"studente"
)); ?>
</div>
but check the parameters... the second one is the URL you need, if it's empty it means current page
This way when you click the button the button is hidden and the page remains with the hidden button
Hope it helps...