How to automatically preset the category for custom users?

Here is my situation:
I have a user role called "student",and the wordpress have a specific category called "event".
While the "student" clicking the "add-post" button(http://my-domian/wp-admin/post-new.php),I want the post to preset the category as "event" instead of let the "student" set the category by himself.
So,can somebody help me to write some codes to finish it?
Thank you very much!