Sending chat (written) messages during lessons is something your teacher will likely do frequently. You can of course respond!
In fact, you can enter the classroom up to 24 hours in advance and send your teacher a message. This is a great way to say hi, or even make a note about something you'd like to cover during the lesson.
And remember, if you're using a mobile device with a small screen, just swipe to the left to use chat!