It's not possible to send a classic message to your students, at the moment there is no inbox functionality. It's only possible to send one collective message that will appear on the home screen for all the students in your class.

You can show multiple messages at the same time:
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In the Teacher environment, click on the Messages tab.

You can select the correct class using a drop-down menu in the top-right of the screen.

If you click on the "New message" button.

You will see an input field where you can write your message. 

Messages can be seen by the entire class; it is not possible to send messages to one student, or a subgroup of students.

Teachers are able to adjust the time the message will be sent out, you can schedule a message in the future. Click 'save' to confirm.

Once your message is saved, it will show up in a list below the "New message" button. Students can now also see your message on the home screen. You now have a few options for your newly written message:

Available: This shows whether or not your message is available to see for your students.
​Edit: This allows you to edit your message.
​E-mail: Clicking this icon will send your message as an e-mail to all the users in your class. It will also show up on the home screen.

It's also possible to send a notification to all your students during a test. Read the article here on how to set this up.