Soon after you begin teaching the class, ask for a volunteer to come up. When someone has come up, ask them to finish teaching the lesson and then sit down. After they have been trying to teach something for a short while, stand up and ask them how they felt.
Ask if it would have been a little bit easier if you had given them some time to prepare their lesson. Then thank the volunteer and ask them to be seated.
Address the class on the importance of being prepared before they go to the temple (sacrament meeting, church, study scriptures etc.)