Teams now has breakout room built into meeting. This new feature will allow you to both automatically or manually create smaller meetings during your class. This form of breakout is good for short in class meetings, the channel-based breakout option is still good if you have a long term group project for the students.
When the breakouts are set up you can make an announcement to all users by selecting the more actions options in the breakout panel.
From here select Make an announcement and type your announcement.
Participants will see this as a notification and will appear as a chat.