Go to the first message you want Archived or deleted and tick the box. 

Now scroll down to the last message you want to archive or delete and hold the Shift key down while clicking the last messages box.  Sometimes I have to click multiple times, but it does work for me.

This should highlight everything if the CTRL + A option is not working.  Once highlighted, select Archive or Delete. 

Personally, I archive them for the record even though they come to my EDU inbox.  This is because by EDU account has a shorter retention policy for emails. 

Cheers,

Ben