The reason why it is currently unavailable to you is because internal messaging is off in your ON system. To turn this on you have to go to:
  1. Core > Settings > Inbox.
  2. Then mark messaging on.
  3. Sign out and sign back in again.
  4. Go to onRecord > Procedures > Section attendance you should not see a small text bubble to the left of take attendance.

Once you select that new option you can now send a message to the teacher and if they have their notifications set it will also send them an email.