If you need to ask your teacher a question privately, you can use the Messaging section to write to them. (Simply click on the Messaging icon on the left sidebar.) You can upload attachments if you need to.
There are two types of messages from your teacher: Class Announcements and a one-on-one conversation.
Class Announcements are one-way messages that are sent to all students. You can't reply to those.
Click on your teacher's name to access your private, one-on-one conversation. Be aware that your teacher can disable replies if you abuse this system, and they’ll likely remove HP from your character if you send in inappropriate message or spam them!