Communicating with a teacher in teams is pretty easy, there are quite a few different ways to do this.
To ask a question, tap on the new post button in the relevant team. This will be seen by everyone in the team and should be relevant to any work set throughout the lesson. You might find that another student is able to answer your question. Make sure when posting a question like this that it is appropriate and that you act responsibly at all times.
To start a chat press ‘Chat’. Tap on the writing icon at the top left of the page. Enter the details of the person you wish to chat with, this should only be a teacher. All chats are recorded and can be accessed by IT staff at school. This is a really quick way to speak to a teacher on your own. You can ask individual questions rather than posting on the general class team. You can also have a chat with multiple people at a time, a teacher might do this to split you into smaller groups.
Teams chat works just like any other messaging app.