Click on Participants, dropdown the filter menu and select Role: Student. Wait until the page reloads.
Go to the end of the students list and click on Select all button.
Dropdown the With selected users… menu and select Send message option.
Writte your message and clic on Send message to X people button.
A copy of the message will be automatically sent to the electronic email that appears into the Profile students.
The original message will be saved into the Messages tool.