The Message dialog

Choose whether to send the message to all connected Students or the currently selected Students. Enter the message (any URLs entered are clickable at the Student). You can choose an icon to be displayed alongside the message from the drop-down list and decide whether to show it at Student machines for a specified time. You can further highlight the message by playing a sound at the Student's machine. Click the Play a sound icon.

If the Student Journal has been started, you will have the option to add the message to this.