Participant list checkbox for bulk actions not labelled for accessability

When viewing the participants list for a course, in either of the two view modes ('brief' or 'user details') the checkbox for bulk user actions has no label on it.

Even for users not relying on accessibility tools it may not be obvious what this checkbox is for and a hover title of "Select <User Name> for bulk action" or similar might be handy and the same text could be used as an aria-label.