There are several modes for Chat Rooms feature. It depends on how your Teacher or admin has set it up for you and your classroom.
Some Teachers prefer Chat Rooms to be used as a “Virtual Notice Board” where only they can post messages and attachments, whereas some prefer Chat Rooms to be used as a common forum where anybody can post.
Your Teacher can instantly convert them into the other. Please talk to your Teacher about how they prefer using the Chat Rooms.