Hi.
Right now this is hardcoded to use default Student role. If you need to change it, you would need to do it in the code:
File: administrator/components/com_joomdle/helpers/content.php
Function: enrolUser
Change 5 by the ID of the role you want to use.
I think we could add an option for a future release, so you could configure which role you want Joomdle to use as student role.