When teacher wants to send a message to all course members, he has to go to Participants List (here a demo example: http://school.demo.moodle.net/user/index.php?id=41), go bottom to the end of the list and press "Select all" button.
If the users list is long - it's placed on more then one page, but "Select all" button affects only those who appear on the page (image attached).
It might mislead the users, and make them mistakenly send the message to only part of users (only those who appears at first page when Participants List divided into pages).
Don't you think?