It is possible to send a message to the whole class during a test. This only works on a whole class level and is not intended for individual messaging.
To send the message, go to the Tests tab and click on the symbol of a text bubble under actions.
Type your message and click on "Send notification" to send it immediately to the whole class.
You can send one message at a time. If you wish to send a different message, you can edit the original message and send a new notification. You can also delete a message.
You can see that there is a message on the test when the text bubble turns purple.
When you send the message, it will immediately show on top of the screen to all students (it's not possible to schedule the message). The students can click on the "X" to close it.